Use DSL style for feature categories in users API

This isn't possible for custom attributes, as they're a mixin, but
everywhere else can use the DSL style.

This also means a slight tweak to the validation in WithFeatureCategory:
previously we didn't allow duplicates at all, but now we allow
duplicates with the same category. That's because the API supports
different HTTP methods to the same path, but those methods will always
be the same category. Omitting the feature category from the duplicates
would look odd.

lib/gitlab/with_feature_category.rb

@@ -25,7 +25,7 @@ module Gitlab
 
       def validate_config!(config)
         empty = config.find { |_, actions| actions.empty? }
-        duplicate_actions = config.values.flatten.group_by(&:itself).select { |_, v| v.count > 1 }.keys
+        duplicate_actions = config.values.map(&:uniq).flatten.group_by(&:itself).select { |_, v| v.count > 1 }.keys
 
         if config.length > 1 && empty
           raise ArgumentError, "#{empty.first} is defined for all actions, but other categories are set"

spec/lib/gitlab/with_feature_category_spec.rb

@@ -56,5 +56,14 @@ RSpec.describe Gitlab::WithFeatureCategory do
         end
       end.to raise_error(ArgumentError, "Actions have multiple feature categories: world")
     end
+
+    it "does not raise an error when multiple calls define the same action and feature category" do
+      expect do
+        Class.new(base_controller) do
+          feature_category :hello, [:world]
+          feature_category :hello, ["world"]
+        end
+      end.not_to raise_error
+    end
   end
 end