Customize the sanitization whitelist only once

Fixes #1651

lib/gitlab/markdown/sanitization_filter.rb

@@ -8,8 +8,10 @@ module Gitlab
     # Extends HTML::Pipeline::SanitizationFilter with a custom whitelist.
     class SanitizationFilter < HTML::Pipeline::SanitizationFilter
       def whitelist
-        whitelist = HTML::Pipeline::SanitizationFilter::WHITELIST
+        whitelist = super
 
+        # Only push these customizations once
+        unless customized?(whitelist[:transformers])
           # Allow code highlighting
           whitelist[:attributes]['pre'] = %w(class)
           whitelist[:attributes]['span'] = %w(class)
@@ -26,10 +28,13 @@ module Gitlab
 
           # Remove `class` attribute from non-highlight spans
           whitelist[:transformers].push(clean_spans)
+        end
 
         whitelist
       end
 
+      private
+
       def remove_rel
         lambda do |env|
           if env[:node_name] == 'a'
@@ -48,6 +53,10 @@ module Gitlab
           end
         end
       end
+
+      def customized?(transformers)
+        transformers.last.source_location[0] == __FILE__
+      end
     end
   end
 end