Remove validates :key, uniqueness due to new validator covers the case

app/models/ci/pipeline_schedule_variable.rb

@@ -4,7 +4,5 @@ module Ci
     include HasVariable
 
     belongs_to :pipeline_schedule
-
-    validates :key, uniqueness: { scope: :pipeline_schedule_id }
   end
 end

spec/controllers/projects/pipeline_schedules_controller_spec.rb

@@ -189,7 +189,6 @@ describe Projects::PipelineSchedulesController do
             key: 'CCC', pipeline_schedule: pipeline_schedule)
         end
 
-        context 'when params include one variable' do
         context 'when adds a new variable' do
           let(:schedule) do
             basic_param.merge({
@@ -205,6 +204,21 @@ describe Projects::PipelineSchedulesController do
           end
         end
 
+        context 'when adds a new duplicated variable' do
+          let(:schedule) do
+            basic_param.merge({
+              variables_attributes: [{ key: 'CCC', value: 'AAA123' }]
+            })
+          end
+
+          it 'returns an error' do
+            expect { go }.not_to change { Ci::PipelineScheduleVariable.count }
+
+            pipeline_schedule.reload
+            expect(assigns(:schedule).errors['variables']).not_to be_empty
+          end
+        end
+
         context 'when updates a variable' do
           let(:schedule) do
             basic_param.merge({
@@ -231,6 +245,22 @@ describe Projects::PipelineSchedulesController do
             expect { go }.to change { Ci::PipelineScheduleVariable.count }.by(-1)
           end
         end
+
+        context 'when deletes and creates a same key simultaneously' do
+          let(:schedule) do
+            basic_param.merge({
+              variables_attributes: [{ id: pipeline_schedule_variable.id, _destroy: true },
+                                     { key: 'CCC', value: 'CCC123' }]
+            })
+          end
+
+          it 'updates the variable' do
+            expect { go }.not_to change { Ci::PipelineScheduleVariable.count }
+
+            pipeline_schedule.reload
+            expect(pipeline_schedule.variables.last.key).to eq('CCC')
+            expect(pipeline_schedule.variables.last.value).to eq('CCC123')
+          end
         end
       end
     end