diff --git a/lib/discourse_diff.rb b/lib/discourse_diff.rb
index c1971ed7108..89d293e4319 100644
--- a/lib/discourse_diff.rb
+++ b/lib/discourse_diff.rb
@@ -184,8 +184,13 @@ 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
-        html_or_text.insert(index_of_class + "class=".length + 1, "diff-#{klass} ")
+        # 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}>"