diff --git a/lib/cli.rb b/lib/cli.rb
index 2a4dd11b2..3f1658566 100644
--- a/lib/cli.rb
+++ b/lib/cli.rb
@@ -14,6 +14,7 @@ require_relative 'mastodon/cache_cli'
 require_relative 'mastodon/upgrade_cli'
 require_relative 'mastodon/email_domain_blocks_cli'
 require_relative 'mastodon/ip_blocks_cli'
+require_relative 'mastodon/maintenance_cli'
 require_relative 'mastodon/version'
 
 module Mastodon
@@ -61,6 +62,9 @@ module Mastodon
     desc 'ip_blocks SUBCOMMAND ...ARGS', 'Manage IP blocks'
     subcommand 'ip_blocks', Mastodon::IpBlocksCLI
 
+    desc 'maintenance SUBCOMMAND ...ARGS', 'Various maintenance utilities'
+    subcommand 'maintenance', Mastodon::MaintenanceCLI
+
     option :dry_run, type: :boolean
     desc 'self-destruct', 'Erase the server from the federation'
     long_desc <<~LONG_DESC
diff --git a/lib/mastodon/maintenance_cli.rb b/lib/mastodon/maintenance_cli.rb
new file mode 100644
index 000000000..191a3b03f
--- /dev/null
+++ b/lib/mastodon/maintenance_cli.rb
@@ -0,0 +1,610 @@
+# frozen_string_literal: true
+
+require 'tty-prompt'
+require_relative '../../config/boot'
+require_relative '../../config/environment'
+require_relative 'cli_helper'
+
+module Mastodon
+  class MaintenanceCLI < Thor
+    include CLIHelper
+
+    def self.exit_on_failure?
+      true
+    end
+
+    MIN_SUPPORTED_VERSION = 2019_10_01_213028
+    MAX_SUPPORTED_VERSION = 2020_10_17_234926
+
+    # Stubs to enjoy ActiveRecord queries while not depending on a particular
+    # version of the code/database
+
+    class Status < ApplicationRecord; end
+    class StatusPin < ApplicationRecord; end
+    class Poll < ApplicationRecord; end
+    class Report < ApplicationRecord; end
+    class Tombstone < ApplicationRecord; end
+    class Favourite < ApplicationRecord; end
+    class Follow < ApplicationRecord; end
+    class FollowRequest < ApplicationRecord; end
+    class Block < ApplicationRecord; end
+    class Mute < ApplicationRecord; end
+    class AccountIdentityProof < ApplicationRecord; end
+    class AccountModerationNote < ApplicationRecord; end
+    class AccountPin < ApplicationRecord; end
+    class ListAccount < ApplicationRecord; end
+    class PollVote < ApplicationRecord; end
+    class Mention < ApplicationRecord; end
+    class AccountDomainBlock < ApplicationRecord; end
+    class AnnouncementReaction < ApplicationRecord; end
+    class FeaturedTag < ApplicationRecord; end
+    class CustomEmoji < ApplicationRecord; end
+    class CustomEmojiCategory < ApplicationRecord; end
+    class Bookmark < ApplicationRecord; end
+    class WebauthnCredential < ApplicationRecord; end
+
+    class PreviewCard < ApplicationRecord
+      self.inheritance_column = false
+    end
+
+    class MediaAttachment < ApplicationRecord
+      self.inheritance_column = nil
+    end
+
+    class AccountStat < ApplicationRecord
+      belongs_to :account, inverse_of: :account_stat
+    end
+
+    class Account < ApplicationRecord
+      # Dummy class, to make migration possible across version changes
+      has_one :user, inverse_of: :account
+      has_one :account_stat, inverse_of: :account
+
+      scope :local, -> { where(domain: nil) }
+
+      def local?
+        domain.nil?
+      end
+
+      def acct
+        local? ? username : "#{username}@#{domain}"
+      end
+    end
+
+    class User < ApplicationRecord
+      belongs_to :account, inverse_of: :user
+    end
+
+    desc 'fix-duplicates', 'Fix duplicates in database and rebuild indexes'
+    long_desc <<~LONG_DESC
+      Delete or merge duplicate accounts, statuses, emojis, etc. and rebuild indexes.
+
+      This is useful if your database indexes are corrupted because of issues such as https://wiki.postgresql.org/wiki/Locale_data_changes
+
+      Mastodon has to be stopped to run this task, which will take a long time and may be destructive.
+    LONG_DESC
+    def fix_duplicates
+      @prompt = TTY::Prompt.new
+
+      if ActiveRecord::Migrator.current_version < MIN_SUPPORTED_VERSION
+        @prompt.warn 'Your version of the database schema is too old and is not supported by this script.'
+        @prompt.warn 'Please update to at least Mastodon 3.0.0 before running this script.'
+        exit(1)
+      elsif ActiveRecord::Migrator.current_version > MAX_SUPPORTED_VERSION
+        @prompt.warn 'Your version of the database schema is more recent than this script, this may cause unexpected errors.'
+        exit(1) unless @prompt.yes?('Continue anyway?')
+      end
+
+      @prompt.warn 'This task will take a long time to run and is potentially destructive.'
+      @prompt.warn 'Please make sure to stop Mastodon and have a backup.'
+      exit(1) unless @prompt.yes?('Continue?')
+
+      deduplicate_accounts!
+      deduplicate_users!
+      deduplicate_account_domain_blocks!
+      deduplicate_account_identity_proofs!
+      deduplicate_announcement_reactions!
+      deduplicate_conversations!
+      deduplicate_custom_emojis!
+      deduplicate_custom_emoji_categories!
+      deduplicate_domain_allows!
+      deduplicate_domain_blocks!
+      deduplicate_unavailable_domains!
+      deduplicate_email_domain_blocks!
+      deduplicate_media_attachments!
+      deduplicate_preview_cards!
+      deduplicate_statuses!
+      deduplicate_tags!
+      deduplicate_webauthn_credentials!
+
+      Rails.cache.clear
+
+      @prompt.say 'Finished!'
+    end
+
+    private
+
+    def deduplicate_accounts!
+      remove_index_if_exists!(:accounts, 'index_accounts_on_username_and_domain_lower')
+
+      @prompt.say 'Deduplicating accounts… for local accounts, you will be asked to chose which account to keep unchanged.'
+
+      find_duplicate_accounts.each do |row|
+        accounts = Account.where(id: row['ids'].split(',')).to_a
+
+        if accounts.first.local?
+          deduplicate_local_accounts!(accounts)
+        else
+          deduplicate_remote_accounts!(accounts)
+        end
+      end
+
+      @prompt.say 'Restoring index_accounts_on_username_and_domain_lower…'
+      if ActiveRecord::Migrator.current_version < 20200620164023
+        ActiveRecord::Base.connection.add_index :accounts, 'lower (username), lower(domain)', name: 'index_accounts_on_username_and_domain_lower', unique: true
+      else
+        ActiveRecord::Base.connection.add_index :accounts, "lower (username), COALESCE(lower(domain), '')", name: 'index_accounts_on_username_and_domain_lower', unique: true
+      end
+    end
+
+    def deduplicate_users!
+      remove_index_if_exists!(:users, 'index_users_on_confirmation_token')
+      remove_index_if_exists!(:users, 'index_users_on_email')
+      remove_index_if_exists!(:users, 'index_users_on_remember_token')
+      remove_index_if_exists!(:users, 'index_users_on_reset_password_token')
+
+      @prompt.say 'Deduplicating user records…'
+
+      # Deduplicating email
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM users GROUP BY email HAVING count(*) > 1").each do |row|
+        users = User.where(id: row['ids'].split(',')).sort_by(&:updated_at).reverse
+        ref_user = users.shift
+        @prompt.warn "Multiple users registered with e-mail address #{ref_user.email}."
+        @prompt.warn "e-mail will be disabled for the following accounts: #{user.map(&:account).map(&:acct).join(', ')}"
+        @prompt.warn 'Please reach out to them and set another address with `tootctl account modify` or delete them.'
+
+        i = 0
+        users.each do |user|
+          user.update!(email: "#{i} " + user.email)
+        end
+      end
+
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM users WHERE confirmation_token IS NOT NULL GROUP BY confirmation_token HAVING count(*) > 1").each do |row|
+        users = User.where(id: row['ids'].split(',')).sort_by(&:created_at).reverse.drop(1)
+        @prompt.warn "Unsetting confirmation token for those accounts: #{users.map(&:account).map(&:acct).join(', ')}"
+
+        users.each do |user|
+          user.update!(confirmation_token: nil)
+        end
+      end
+
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM users WHERE remember_token IS NOT NULL GROUP BY remember_token HAVING count(*) > 1").each do |row|
+        users = User.where(id: row['ids'].split(',')).sort_by(&:updated_at).reverse.drop(1)
+        @prompt.warn "Unsetting remember token for those accounts: #{users.map(&:account).map(&:acct).join(', ')}"
+
+        users.each do |user|
+          user.update!(remember_token: nil)
+        end
+      end
+
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM users WHERE reset_password_token IS NOT NULL GROUP BY reset_password_token HAVING count(*) > 1").each do |row|
+        users = User.where(id: row['ids'].split(',')).sort_by(&:updated_at).reverse.drop(1)
+        @prompt.warn "Unsetting password reset token for those accounts: #{users.map(&:account).map(&:acct).join(', ')}"
+
+        users.each do |user|
+          user.update!(reset_password_token: nil)
+        end
+      end
+
+      @prompt.say 'Restoring users indexes…'
+      ActiveRecord::Base.connection.add_index :users, ['confirmation_token'], name: 'index_users_on_confirmation_token', unique: true
+      ActiveRecord::Base.connection.add_index :users, ['email'], name: 'index_users_on_email', unique: true
+      ActiveRecord::Base.connection.add_index :users, ['remember_token'], name: 'index_users_on_remember_token', unique: true
+      ActiveRecord::Base.connection.add_index :users, ['reset_password_token'], name: 'index_users_on_reset_password_token', unique: true
+    end
+
+    def deduplicate_account_domain_blocks!
+      remove_index_if_exists!(:account_domain_blocks, 'index_account_domain_blocks_on_account_id_and_domain')
+
+      @prompt.say 'Removing duplicate account domain blocks…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM account_domain_blocks GROUP BY account_id, domain HAVING count(*) > 1").each do |row|
+        AccountDomainBlock.where(id: row['ids'].split(',').drop(1)).delete_all
+      end
+
+      @prompt.say 'Restoring account domain blocks indexes…'
+      ActiveRecord::Base.connection.add_index :account_domain_blocks, ['account_id', 'domain'], name: 'index_account_domain_blocks_on_account_id_and_domain', unique: true
+    end
+
+    def deduplicate_account_identity_proofs!
+      remove_index_if_exists!(:account_identity_proofs, 'index_account_proofs_on_account_and_provider_and_username')
+
+      @prompt.say 'Removing duplicate account identity proofs…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM account_identity_proofs GROUP BY account_id, provider, provider_username HAVING count(*) > 1").each do |row|
+        AccountIdentityProof.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring account identity proofs indexes…'
+      ActiveRecord::Base.connection.add_index :account_identity_proofs, ['account_id', 'provider', 'provider_username'], name: 'index_account_proofs_on_account_and_provider_and_username', unique: true
+    end
+
+    def deduplicate_announcement_reactions!
+      return unless ActiveRecord::Base.connection.table_exists?(:announcement_reactions)
+
+      remove_index_if_exists!(:announcement_reactions, 'index_announcement_reactions_on_account_id_and_announcement_id')
+
+      @prompt.say 'Removing duplicate account identity proofs…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM announcement_reactions GROUP BY account_id, announcement_id, name HAVING count(*) > 1").each do |row|
+        AnnouncementReaction.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring announcement_reactions indexes…'
+      ActiveRecord::Base.connection.add_index :announcement_reactions, ['account_id', 'announcement_id', 'name'], name: 'index_announcement_reactions_on_account_id_and_announcement_id', unique: true
+    end
+
+    def deduplicate_conversations!
+      remove_index_if_exists!(:conversations, 'index_conversations_on_uri')
+
+      @prompt.say 'Deduplicating conversations…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM conversations WHERE uri IS NOT NULL GROUP BY uri HAVING count(*) > 1").each do |row|
+        conversations = Conversation.where(id: row['ids'].split(',')).sort_by(&:id).reverse
+
+        ref_conversation = conversations.shift
+
+        conversations.each do |other|
+          merge_conversations!(ref_conversation, other)
+          other.destroy
+        end
+      end
+
+      @prompt.say 'Restoring conversations indexes…'
+      ActiveRecord::Base.connection.add_index :conversations, ['uri'], name: 'index_conversations_on_uri', unique: true
+    end
+
+    def deduplicate_custom_emojis!
+      remove_index_if_exists!(:custom_emojis, 'index_custom_emojis_on_shortcode_and_domain')
+
+      @prompt.say 'Deduplicating custom_emojis…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM custom_emojis GROUP BY shortcode, domain HAVING count(*) > 1").each do |row|
+        emojis = CustomEmoji.where(id: row['ids'].split(',')).sort_by(&:id).reverse
+
+        ref_emoji = emojis.shift
+
+        emojis.each do |other|
+          merge_custom_emojis!(ref_emoji, other)
+          other.destroy
+        end
+      end
+
+      @prompt.say 'Restoring custom_emojis indexes…'
+      ActiveRecord::Base.connection.add_index :custom_emojis, ['shortcode', 'domain'], name: 'index_custom_emojis_on_shortcode_and_domain', unique: true
+    end
+
+    def deduplicate_custom_emoji_categories!
+      remove_index_if_exists!(:custom_emoji_categories, 'index_custom_emoji_categories_on_name')
+
+      @prompt.say 'Deduplicating custom_emoji_categories…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM custom_emoji_categories GROUP BY name HAVING count(*) > 1").each do |row|
+        categories = CustomEmojiCategory.where(id: row['ids'].split(',')).sort_by(&:id).reverse
+
+        ref_category = categories.shift
+
+        categories.each do |other|
+          merge_custom_emoji_categories!(ref_category, other)
+          other.destroy
+        end
+      end
+
+      @prompt.say 'Restoring custom_emoji_categories indexes…'
+      ActiveRecord::Base.connection.add_index :custom_emoji_categories, ['name'], name: 'index_custom_emoji_categories_on_name', unique: true
+    end
+
+    def deduplicate_domain_allows!
+      remove_index_if_exists!(:domain_allows, 'index_domain_allows_on_domain')
+
+      @prompt.say 'Deduplicating domain_allows…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM domain_allows GROUP BY domain HAVING count(*) > 1").each do |row|
+        DomainAllow.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring domain_allows indexes…'
+      ActiveRecord::Base.connection.add_index :domain_allows, ['domain'], name: 'index_domain_allows_on_domain', unique: true
+    end
+
+    def deduplicate_domain_blocks!
+      remove_index_if_exists!(:domain_blocks, 'index_domain_blocks_on_domain')
+
+      @prompt.say 'Deduplicating domain_allows…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM domain_blocks GROUP BY domain HAVING count(*) > 1").each do |row|
+        domain_blocks = DomainBlock.where(id: row['ids'].split(',')).by_severity.reverse.to_a
+
+        reject_media = domain_blocks.any?(&:reject_media?)
+        reject_reports = domain_blocks.any?(&:reject_reports?)
+
+        reference_block = domain_blocks.shift
+
+        private_comment = domain_blocks.reduce(reference_block.private_comment.presence) { |a, b| a || b.private_comment.presence }
+        public_comment  = domain_blocks.reduce(reference_block.public_comment.presence)  { |a, b| a || b.public_comment.presence }
+
+        reference_block.update!(reject_media: reject_media, reject_reports: reject_reports, private_comment: private_comment, public_comment: public_comment)
+
+        domain_blocks.each(&:destroy)
+      end
+
+      @prompt.say 'Restoring domain_blocks indexes…'
+      ActiveRecord::Base.connection.add_index :domain_blocks, ['domain'], name: 'index_domain_blocks_on_domain', unique: true
+    end
+
+    def deduplicate_unavailable_domains!
+      return unless ActiveRecord::Base.connection.table_exists?(:unavailable_domains)
+
+      remove_index_if_exists!(:unavailable_domains, 'index_unavailable_domains_on_domain')
+
+      @prompt.say 'Deduplicating unavailable_domains…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM unavailable_domains GROUP BY domain HAVING count(*) > 1").each do |row|
+        UnavailableDomain.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring domain_allows indexes…'
+      ActiveRecord::Base.connection.add_index :unavailable_domains, ['domain'], name: 'index_unavailable_domains_on_domain', unique: true
+    end
+
+    def deduplicate_email_domain_blocks!
+      remove_index_if_exists!(:email_domain_blocks, 'index_email_domain_blocks_on_domain')
+
+      @prompt.say 'Deduplicating email_domain_blocks…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM email_domain_blocks GROUP BY domain HAVING count(*) > 1").each do |row|
+        domain_blocks = EmailDomainBlock.where(id: row['ids'].split(',')).sort_by { |b| b.parent.nil? ? 1 : 0 }.to_a
+        domain_blocks.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring email_domain_blocks indexes…'
+      ActiveRecord::Base.connection.add_index :email_domain_blocks, ['domain'], name: 'index_email_domain_blocks_on_domain', unique: true
+    end
+
+    def deduplicate_media_attachments!
+      remove_index_if_exists!(:media_attachments, 'index_media_attachments_on_shortcode')
+
+      @prompt.say 'Deduplicating media_attachments…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM media_attachments WHERE shortcode IS NOT NULL GROUP BY shortcode HAVING count(*) > 1").each do |row|
+        MediaAttachment.where(id: row['ids'].split(',').drop(1)).update_all(shortcode: nil)
+      end
+
+      @prompt.say 'Restoring media_attachments indexes…'
+      ActiveRecord::Base.connection.add_index :media_attachments, ['shortcode'], name: 'index_media_attachments_on_shortcode', unique: true
+    end
+
+    def deduplicate_preview_cards!
+      remove_index_if_exists!(:preview_cards, 'index_preview_cards_on_url')
+
+      @prompt.say 'Deduplicating preview_cards…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM preview_cards GROUP BY url HAVING count(*) > 1").each do |row|
+        PreviewCard.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring preview_cards indexes…'
+      ActiveRecord::Base.connection.add_index :preview_cards, ['url'], name: 'index_preview_cards_on_url', unique: true
+    end
+
+    def deduplicate_statuses!
+      remove_index_if_exists!(:statuses, 'index_statuses_on_uri')
+
+      @prompt.say 'Deduplicating statuses…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM statuses WHERE uri IS NOT NULL GROUP BY uri HAVING count(*) > 1").each do |row|
+        statuses = Status.where(id: row['ids'].split(',')).sort_by(&:id)
+        ref_status = statuses.shift
+        statuses.each do |status|
+          merge_statuses!(ref_status, status) if status.account_id == ref_status.account_id
+          status.destroy
+        end
+      end
+
+      @prompt.say 'Restoring statuses indexes…'
+      ActiveRecord::Base.connection.add_index :statuses, ['uri'], name: 'index_statuses_on_uri', unique: true
+    end
+
+    def deduplicate_tags!
+      remove_index_if_exists!(:tags, 'index_tags_on_name_lower')
+
+      @prompt.say 'Deduplicating tags…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM tags GROUP BY lower((name)::text) HAVING count(*) > 1").each do |row|
+        tags = Tag.where(id: row['ids'].split(',')).sort_by { |t| [t.usable?, t.trendable?, t.listable?].count(false) }
+        ref_tag = tags.shift
+        tags.each do |tag|
+          merge_tags!(ref_tag, tag)
+          tag.destroy
+        end
+      end
+
+      @prompt.say 'Restoring tags indexes…'
+      ActiveRecord::Base.connection.add_index :tags, 'lower((name)::text)', name: 'index_tags_on_name_lower', unique: true
+    end
+
+    def deduplicate_webauthn_credentials!
+      return unless ActiveRecord::Base.connection.table_exists?(:webauthn_credentials)
+
+      remove_index_if_exists!(:webauthn_credentials, 'index_webauthn_credentials_on_external_id')
+
+      @prompt.say 'Deduplicating webauthn_credentials…'
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM webauthn_credentials GROUP BY external_id HAVING count(*) > 1").each do |row|
+        WebauthnCredential.where(id: row['ids'].split(',')).sort_by(&:id).reverse.drop(1).each(&:destroy)
+      end
+
+      @prompt.say 'Restoring webauthn_credentials indexes…'
+      ActiveRecord::Base.connection.add_index :webauthn_credentials, ['external_id'], name: 'index_webauthn_credentials_on_external_id', unique: true
+    end
+
+    def deduplicate_local_accounts!(accounts)
+      accounts = accounts.sort_by(&:id).reverse
+
+      @prompt.warn "Multiple local accounts were found for username '#{accounts.first.username}'."
+      @prompt.warn 'All those accounts are distinct accounts but only the most recently-created one is fully-functionnal.'
+
+      accounts.each_with_index do |account, idx|
+        @prompt.say '%2d. %s: created at: %s; updated at: %s; last logged in at: %s; statuses: %5d; last status at: %s' % [idx, account.username, account.created_at, account.updated_at, account.user&.last_sign_in_at&.to_s || 'N/A', account.account_stat&.statuses_count || 0, account.account_stat&.last_status_at || 'N/A']
+      end
+
+      @prompt.say 'Please chose the one to keep unchanged, other ones will be automatically renamed.'
+
+      ref_id = @prompt.ask('Account to keep unchanged:') do |q|
+        q.required true
+        q.default 0
+        q.convert :int
+      end
+
+      accounts.delete_at(ref_id)
+
+      i = 0
+      accounts.each do |account|
+        i += 1
+        username = account.username + "_#{i}"
+
+        while Account.local.exists?(username: username)
+          i += 1
+          username = account.username + "_#{i}"
+        end
+
+        account.update!(username: username)
+      end
+    end
+
+    def deduplicate_remote_accounts!(accounts)
+      accounts = accounts.sort_by(&:updated_at).reverse
+
+      reference_account = accounts.shift
+
+      accounts.each do |other_account|
+        if other_account.public_key == reference_account.public_key
+          # The accounts definitely point to the same resource, so
+          # it's safe to re-attribute content and relationships
+          merge_accounts!(reference_account, other_account)
+        end
+
+        other_account.destroy
+      end
+    end
+
+    def merge_accounts!(main_account, duplicate_account)
+      # Since it's the same remote resource, the remote resource likely
+      # already believes we are following/blocking, so it's safe to
+      # re-attribute the relationships too. However, during the presence
+      # of the index bug users could have *also* followed the reference
+      # account already, therefore mass update will not work and we need
+      # to check for (and skip past) uniqueness errors
+      owned_classes = [
+        Status, StatusPin, MediaAttachment, Poll, Report, Tombstone, Favourite,
+        Follow, FollowRequest, Block, Mute, AccountIdentityProof,
+        AccountModerationNote, AccountPin, AccountStat, ListAccount,
+        PollVote, Mention
+      ]
+      owned_classes.each do |klass|
+        klass.where(account_id: duplicate_account.id).find_each do |record|
+          begin
+            record.update_attribute(:account_id, main_account.id)
+          rescue ActiveRecord::RecordNotUnique
+            next
+          end
+        end
+      end
+
+      target_classes = [Follow, FollowRequest, Block, Mute, AccountModerationNote, AccountPin]
+      target_classes.each do |klass|
+        klass.where(target_account_id: duplicate_account.id).find_each do |record|
+          begin
+            record.update_attribute(:target_account_id, main_account.id)
+          rescue ActiveRecord::RecordNotUnique
+            next
+          end
+        end
+      end
+    end
+
+    def merge_conversations!(main_conv, duplicate_conv)
+      owned_classes = [ConversationMute, AccountConversation]
+      owned_classes.each do |klass|
+        klass.where(conversation_id: duplicate_conv.id).find_each do |record|
+          begin
+            record.update_attribute(:account_id, main_conv.id)
+          rescue ActiveRecord::RecordNotUnique
+            next
+          end
+        end
+      end
+    end
+
+    def merge_custom_emojis!(main_emoji, duplicate_emoji)
+      owned_classes = [AnnouncementReaction]
+      owned_classes.each do |klass|
+        klass.where(custom_emoji_id: duplicate_emoji.id).update_all(custom_emoji_id: main_emoji.id)
+      end
+    end
+
+    def merge_custom_emoji_categories!(main_category, duplicate_category)
+      owned_classes = [CustomEmoji]
+      owned_classes.each do |klass|
+        klass.where(category_id: duplicate_category.id).update_all(category_id: main_category.id)
+      end
+    end
+
+    def merge_statuses!(main_status, duplicate_status)
+      owned_classes = [Favourite, Mention, Poll]
+      owned_classes << Bookmark if ActiveRecord::Base.connection.table_exists?(:bookmarks)
+      owned_classes.each do |klass|
+        klass.where(status_id: duplicate_status.id).find_each do |record|
+          begin
+            record.update_attribute(:status_id, main_status.id)
+          rescue ActiveRecord::RecordNotUnique
+            next
+          end
+        end
+      end
+
+      StatusPin.where(account_id: main_status.account_id, status_id: duplicate_status.id).find_each do |record|
+        begin
+          record.update_attribute(:status_id, main_status.id)
+        rescue ActiveRecord::RecordNotUnique
+          next
+        end
+      end
+
+      Status.where(in_reply_to_id: duplicate_status.id).find_each do |record|
+        begin
+          record.update_attribute(:in_reply_to_id, main_status.id)
+        rescue ActiveRecord::RecordNotUnique
+          next
+        end
+      end
+
+      Status.where(reblog_of_id: duplicate_status.id).find_each do |record|
+        begin
+          record.update_attribute(:reblog_of_id, main_status.id)
+        rescue ActiveRecord::RecordNotUnique
+          next
+        end
+      end
+    end
+
+    def merge_tags!(main_tag, duplicate_tag)
+      [FeaturedTag].each do |klass|
+        klass.where(tag_id: duplicate_tag.id).find_each do |record|
+          begin
+            record.update_attribute(:tag_id, main_tag.id)
+          rescue ActiveRecord::RecordNotUnique
+            next
+          end
+        end
+      end
+    end
+
+    def find_duplicate_accounts
+      ActiveRecord::Base.connection.select_all("SELECT string_agg(id::text, ',') AS ids FROM accounts GROUP BY lower(username), COALESCE(lower(domain), '') HAVING count(*) > 1")
+    end
+
+    def remove_index_if_exists!(table, name)
+      ActiveRecord::Base.connection.remove_index(table, name: name)
+    rescue ArgumentError
+      nil
+    rescue ActiveRecord::StatementInvalid
+      nil
+    end
+  end
+end
diff --git a/lib/tasks/db.rake b/lib/tasks/db.rake
index b76e90131..199155107 100644
--- a/lib/tasks/db.rake
+++ b/lib/tasks/db.rake
@@ -48,6 +48,17 @@ namespace :db do
     end
   end
 
+  task :post_migration_hook do
+    at_exit do
+      unless %w(C POSIX).include?(ActiveRecord::Base.connection.execute('SELECT datcollate FROM pg_database WHERE datname = current_database();').first['datcollate'])
+        Rails.logger.warn 'WARNING: Your database is using an unsafe collation setting, which might result in index corruption.'
+        Rails.logger.warn 'WARNING: See https://docs.joinmastodon.org/admin/troubleshooting/index-corruption/#am-i-affected'
+      end
+    end
+  end
+
+  Rake::Task['db:migrate'].enhance(['db:post_migration_hook'])
+
   # Before we load the schema, define the timestamp_id function.
   # Idiomatically, we might do this in a migration, but then it
   # wouldn't end up in schema.rb, so we'd need to figure out a way to