Validate project variables by scope

app/models/project.rb

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