Change ConfigLoader#added_features to be a Set.

lib/rubocop/config_loader.rb

@@ -25,13 +25,14 @@ class << self
       attr_accessor :debug, :ignore_parent_exclusion,
                     :disable_pending_cops, :enable_pending_cops
       attr_writer :default_configuration, :project_root
+      attr_reader :loaded_features
 
       alias debug? debug
       alias ignore_parent_exclusion? ignore_parent_exclusion
 
       def clear_options
         @debug = nil
-        @loaded_features = []
+        @loaded_features = Set.new
         FileFinder.root_level = nil
       end
 
@@ -174,19 +175,11 @@ def merge_with_default(config, config_file, unset_nil: true)
         resolver.merge_with_default(config, config_file, unset_nil: unset_nil)
       end
 
-      def loaded_features
-        @loaded_features.flatten.compact.uniq
-      end
-
       # @api private
       # Used to add features that were required inside a config or from
       # the CLI using `--require`.
       def add_loaded_features(loaded_features)
-        if instance_variable_defined?(:@loaded_features)
-          instance_variable_get(:@loaded_features) << loaded_features
-        else
-          instance_variable_set(:@loaded_features, [loaded_features])
-        end
+        @loaded_features.merge(Array(loaded_features))
       end
 
       private

spec/rubocop/cli/cli_options_spec.rb

@@ -177,6 +177,32 @@
       end
     end
 
+    context 'when requiring extension cops in multiple layers' do
+      before do
+        create_file('.rubocop-parent.yml', <<~YAML)
+          require:
+            - rubocop-performance
+        YAML
+
+        create_file('.rubocop.yml', <<~YAML)
+          inherit_from: ./.rubocop-parent.yml
+          require:
+            - rubocop-rspec
+        YAML
+      end
+
+      it 'shows with version of extension cops' do
+        # Run in different process that requiring rubocop-performance and rubocop-rspec
+        # does not affect other testing processes.
+        output = `ruby -I . "#{rubocop}" -V --disable-pending-cops`
+        expect(output).to include(RuboCop::Version::STRING)
+        expect(output).to match(/Parser \d+\.\d+\.\d+/)
+        expect(output).to match(/rubocop-ast \d+\.\d+\.\d+/)
+        expect(output).to match(/rubocop-performance \d+\.\d+\.\d+/)
+        expect(output).to match(/rubocop-rspec \d+\.\d+\.\d+/)
+      end
+    end
+
     context 'when requiring redundant extension cop' do
       before do
         create_file('ext.yml', <<~YAML)