Use DISTINCT in group runner query for RegisterJobService

app/services/ci/register_job_service.rb

@@ -278,7 +278,7 @@ module Ci
       # Workaround for weird Rails bug, that makes `runner.groups.to_sql` to return `runner_id = NULL`
       groups = ::Group.joins(:runner_namespaces).merge(runner.runner_namespaces)
 
-      hierarchy_groups = Gitlab::ObjectHierarchy.new(groups).base_and_descendants
+      hierarchy_groups = Gitlab::ObjectHierarchy.new(groups, options: { use_distinct: Feature.enabled?(:use_distinct_in_register_job_object_hierarchy) }).base_and_descendants
       projects = Project.where(namespace_id: hierarchy_groups)
         .with_group_runners_enabled
         .with_builds_enabled

config/feature_flags/development/use_distinct_in_register_job_object_hierarchy.yml

+---
+name: use_distinct_in_register_job_object_hierarchy
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57045
+rollout_issue_url:
+milestone: '13.11'
+type: development
+group: group::continuous integration
+default_enabled: false

spec/services/ci/register_job_service_spec.rb

@@ -225,6 +225,26 @@ module Ci
             end
           end
 
+          context 'when the use_distinct_in_register_job_object_hierarchy feature flag is enabled' do
+            before do
+              stub_feature_flags(use_distinct_in_register_job_object_hierarchy: true)
+            end
+
+            it 'calls DISTINCT' do
+              expect(described_class.new(group_runner).send(:builds_for_group_runner).to_sql).to include("DISTINCT")
+            end
+          end
+
+          context 'when the use_distinct_in_register_job_object_hierarchy feature flag is disabled' do
+            before do
+              stub_feature_flags(use_distinct_in_register_job_object_hierarchy: false)
+            end
+
+            it 'does not call DISTINCT' do
+              expect(described_class.new(group_runner).send(:builds_for_group_runner).to_sql).not_to include("DISTINCT")
+            end
+          end
+
           context 'group runner' do
             let(:build) { execute(group_runner) }