Improve validation message and add changelog

app/validators/variable_duplicates_validator.rb

@@ -20,7 +20,7 @@ class VariableDuplicatesValidator < ActiveModel::EachValidator
   def validate_duplicates(record, attribute, values)
     duplicates = values.reject(&:marked_for_destruction?).group_by(&:key).select { |_, v| v.many? }.map(&:first)
     if duplicates.any?
-      error_message = "has duplicate variables (#{duplicates.join(", ")})"
+      error_message = "have duplicate values (#{duplicates.join(", ")})"
       error_message += " for #{values.first.send(options[:scope])} scope" if options[:scope] # rubocop:disable GitlabSecurity/PublicSend
       record.errors.add(attribute, error_message)
     end

changelogs/unreleased/fix-validation-of-environment-scope-for-variables.yml

+---
+title: Fix validation of environment scope of variables
+merge_request:
+author:
+type: fixed

spec/support/features/variable_list_shared_examples.rb

@@ -263,7 +263,7 @@ shared_examples 'variable list' do
 
     # We check the first row because it re-sorts to alphabetical order on refresh
     page.within('.js-ci-variable-list-section') do
-      expect(find('.js-ci-variable-error-box')).to have_content(/Validation failed Variables has duplicate variables \(.+\)/)
+      expect(find('.js-ci-variable-error-box')).to have_content(/Validation failed Variables have duplicate values \(.+\)/)
     end
   end
 end