Take a look at this .LRN forum thread (http://openacs.org/forums/message-view?message_id=242134
), and click on an individual post reply link toward the end of the thread. You'll see the Subject listed as something like "Re Re Re Re...". It looks like forums is just blindly preprending Re to the existing subject without regard to an existing Re.
The email notification's subject field gets longer and longer.