Fix duplicated approvers scenario

Prevents duplicated record.

ee/app/models/ee/merge_request.rb

@@ -92,7 +92,7 @@ module EE
           rule = approval_rules.code_owner.first
           rule ||= approval_rules.code_owner.create!(name: ApprovalMergeRequestRule::DEFAULT_NAME_FOR_CODE_OWNER)
 
-          rule.users = code_owners
+          rule.users = code_owners.uniq
         end
       else
         approval_rules.code_owner.delete_all

ee/lib/gitlab/background_migration/migrate_approver_to_approval_rules.rb

@@ -56,11 +56,11 @@ module Gitlab
         has_many :approval_rules, class_name: 'ApprovalMergeRequestRule'
 
         def approver_ids
-          @approver_ids ||= Approver.where(target_type: 'MergeRequest', target_id: id).pluck(:user_id)
+          @approver_ids ||= Approver.where(target_type: 'MergeRequest', target_id: id).pluck('distinct user_id')
         end
 
         def approver_group_ids
-          @approver_group_ids ||= ApproverGroup.where(target_type: 'MergeRequest', target_id: id).pluck(:group_id)
+          @approver_group_ids ||= ApproverGroup.where(target_type: 'MergeRequest', target_id: id).pluck('distinct group_id')
         end
 
         def sync_code_owners_with_approvers
@@ -78,11 +78,11 @@ module Gitlab
         has_many :approval_rules, class_name: 'ApprovalProjectRule'
 
         def approver_ids
-          @approver_ids ||= Approver.where(target_type: 'Project', target_id: id).pluck(:user_id)
+          @approver_ids ||= Approver.where(target_type: 'Project', target_id: id).pluck('distinct user_id')
         end
 
         def approver_group_ids
-          @approver_group_ids ||= ApproverGroup.where(target_type: 'Project', target_id: id).pluck(:group_id)
+          @approver_group_ids ||= ApproverGroup.where(target_type: 'Project', target_id: id).pluck('distinct group_id')
         end
       end
 

ee/spec/lib/gitlab/background_migration/migrate_approver_to_approval_rules_spec.rb

@@ -86,8 +86,10 @@ describe Gitlab::BackgroundMigration::MigrateApproverToApprovalRules do
       context 'when project rule is present' do
         let!(:project_rule) { create(:approval_project_rule, project: target.target_project) }
 
-        it "sets MR rule's source to project rule" do
-          create_member_in(create(:user), :old_schema)
+        it "sets MR rule's source to project rule without duplication" do
+          user = create(:user)
+          create_member_in(user, :old_schema)
+          create_member_in(user, :old_schema)
 
           described_class.new.perform(target_type, target.id)