Remove duplicates on migration

db/migrate/20220929114423_add_unique_index_to_ldap_groups_membership.rb

@@ -1,5 +1,17 @@
 class AddUniqueIndexToLdapGroupsMembership < ActiveRecord::Migration[7.0]
   def change
+    reversible do |dir|
+      dir.up { remove_duplicate_memberships! }
+    end
+
     add_index :ldap_groups_memberships, %i[user_id group_id], unique: true
   end
+
+  def remove_duplicate_memberships!
+    ActiveRecord::Base.connection.execute <<~SQL.squish
+      DELETE FROM ldap_groups_memberships m1
+      USING ldap_groups_memberships m2
+      WHERE m1.id < m2.id AND m1.user_id = m2.user_id AND m1.group_id = m2.group_id;
+    SQL
+  end
 end