FIX: don't add diff classes more than once

lib/discourse_diff.rb

@@ -184,9 +184,14 @@ class DiscourseDiff
         # add it right before the ">"
         html_or_text.insert(index_of_next_chevron, " class=\"diff-#{klass}\"")
       else
-        # we have a class, insert it at the beginning
+        # we have a class, insert it at the beginning if not already present
+        classes = html_or_text[/class=(["'])([^\1]*)\1/, 2]
+        if classes.include?("diff-#{klass}")
+          html_or_text
+        else
           html_or_text.insert(index_of_class + "class=".length + 1, "diff-#{klass} ")
         end
+      end
     else
       "<#{klass}>#{html_or_text}</#{klass}>"
     end