Fix validation of duplicate new variables

app/models/group.rb

@@ -36,6 +36,7 @@ class Group < Namespace
   validate :visibility_level_allowed_by_projects
   validate :visibility_level_allowed_by_sub_groups
   validate :visibility_level_allowed_by_parent
+  validates :variables, variable_duplicates: true
 
   validates :two_factor_grace_period, presence: true, numericality: { greater_than_or_equal_to: 0 }
 

app/models/project.rb

@@ -261,6 +261,7 @@ class Project < ActiveRecord::Base
   validates :repository_storage,
     presence: true,
     inclusion: { in: ->(_object) { Gitlab.config.repositories.storages.keys } }
+  validates :variables, variable_duplicates: true
 
   has_many :uploads, as: :model, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 

spec/support/shared_examples/controllers/variables_shared_examples.rb

@@ -48,6 +48,29 @@ shared_examples 'PATCH #update updates variables' do
     end
   end
 
+  context 'with duplicate new variable parameters' do
+    let(:variables_attributes) do
+      [
+        new_variable_attributes,
+        new_variable_attributes.merge(value: 'other_value')
+      ]
+    end
+
+    it 'does not update the existing variable' do
+      expect { subject }.not_to change { variable.reload.value }
+    end
+
+    it 'does not create the new variable' do
+      expect { subject }.not_to change { owner.variables.count }
+    end
+
+    it 'returns a bad request response' do
+      subject
+
+      expect(response).to have_gitlab_http_status(:bad_request)
+    end
+  end
+
   context 'with valid new variable parameters' do
     let(:variables_attributes) do
       [