diff --git a/lib/mastodon/cli/maintenance.rb b/lib/mastodon/cli/maintenance.rb
index 2e5243468b..98067c6e34 100644
--- a/lib/mastodon/cli/maintenance.rb
+++ b/lib/mastodon/cli/maintenance.rb
@@ -546,7 +546,7 @@ module Mastodon::CLI
       if migrator_version < 2021_04_21_121431
         ActiveRecord::Base.connection.add_index :tags, 'lower((name)::text)', name: 'index_tags_on_name_lower', unique: true
       else
-        ActiveRecord::Base.connection.execute 'CREATE UNIQUE INDEX CONCURRENTLY index_tags_on_name_lower_btree ON tags (lower(name) text_pattern_ops)'
+        ActiveRecord::Base.connection.execute 'CREATE UNIQUE INDEX index_tags_on_name_lower_btree ON tags (lower(name) text_pattern_ops)'
       end
     end
 
diff --git a/spec/lib/mastodon/cli/maintenance_spec.rb b/spec/lib/mastodon/cli/maintenance_spec.rb
index 02169b7a42..61331e6dba 100644
--- a/spec/lib/mastodon/cli/maintenance_spec.rb
+++ b/spec/lib/mastodon/cli/maintenance_spec.rb
@@ -52,5 +52,78 @@ describe Mastodon::CLI::Maintenance do
           .and raise_error(SystemExit)
       end
     end
+
+    context 'when requirements are met' do
+      before do
+        allow(ActiveRecord::Migrator).to receive(:current_version).and_return(2023_08_22_081029) # The latest migration before the cutoff
+        agree_to_backup_warning
+      end
+
+      context 'with duplicate accounts' do
+        before do
+          prepare_duplicate_data
+        end
+
+        let(:duplicate_account_username) { 'username' }
+        let(:duplicate_account_domain) { 'host.example' }
+
+        it 'runs the deduplication process' do
+          expect { subject }
+            .to output_results(
+              'Deduplicating accounts',
+              'Restoring index_accounts_on_username_and_domain_lower',
+              'Reindexing textual indexes on accounts…',
+              'Finished!'
+            )
+            .and change(duplicate_accounts, :count).from(2).to(1)
+        end
+
+        def duplicate_accounts
+          Account.where(username: duplicate_account_username, domain: duplicate_account_domain)
+        end
+
+        def prepare_duplicate_data
+          ActiveRecord::Base.connection.remove_index :accounts, name: :index_accounts_on_username_and_domain_lower
+          Fabricate(:account, username: duplicate_account_username, domain: duplicate_account_domain)
+          Fabricate.build(:account, username: duplicate_account_username, domain: duplicate_account_domain).save(validate: false)
+        end
+      end
+
+      context 'with duplicate users' do
+        before do
+          prepare_duplicate_data
+        end
+
+        let(:duplicate_email) { 'duplicate@example.host' }
+
+        it 'runs the deduplication process' do
+          expect { subject }
+            .to output_results(
+              'Deduplicating user records',
+              'Restoring users indexes',
+              'Finished!'
+            )
+            .and change(duplicate_users, :count).from(2).to(1)
+        end
+
+        def duplicate_users
+          User.where(email: duplicate_email)
+        end
+
+        def prepare_duplicate_data
+          ActiveRecord::Base.connection.remove_index :users, :email
+          Fabricate(:user, email: duplicate_email)
+          Fabricate.build(:user, email: duplicate_email).save(validate: false)
+        end
+      end
+
+      def agree_to_backup_warning
+        allow(cli.shell)
+          .to receive(:yes?)
+          .with('Continue? (Yes/No)')
+          .and_return(true)
+          .once
+      end
+    end
   end
 end