add a uniq constraints on issues and mrs labels

app/services/issuable_base_service.rb

@@ -113,7 +113,7 @@ class IssuableBaseService < BaseService
       new_label_ids -= remove_label_ids if remove_label_ids
     end
-    new_label_ids
+    new_label_ids.uniq
   end
   def available_labels

changelogs/unreleased/53210-add-uniq-constraints-on-issues-and-mrs-labels.yml

+---
+title: add a uniq constraints on issues and mrs labels
+merge_request: 25435
+author: Antoine Huret
+type: fixed

spec/services/issues/create_service_spec.rb

@@ -138,6 +138,20 @@ describe Issues::CreateService do
         end
       end
+      context 'when duplicate label titles are given' do
+        let(:label) { create(:label, project: project) }
+
+        let(:opts) do
+          { title: 'Title',
+            description: 'Description',
+            labels: [label.title, label.title] }
+        end
+
+        it 'assigns the label once' do
+          expect(issue.labels).to contain_exactly(label)
+        end
+      end
+
       it 'executes issue hooks when issue is not confidential' do
         opts = { title: 'Title', description: 'Description', confidential: false }

spec/services/issues/update_service_spec.rb

@@ -592,6 +592,16 @@ describe Issues::UpdateService, :mailer do
           expect(result.label_ids).not_to include(label.id)
         end
       end
+
+      context 'when duplicate label titles are given' do
+        let(:params) do
+          { labels: [label3.title, label3.title] }
+        end
+
+        it 'assigns the label once' do
+          expect(result.labels).to contain_exactly(label3)
+        end
+      end
     end
     context 'updating asssignee_id' do