That's a new one, and something I can't duplicate at the moment. Have you added any new forums recently? I'll take a look through the code to see if I can figure out what would cause this.
I can't reproduce it, so I'm having problems figuring out what it is. I've even reuploaded the product file to your server and its still duplicating. I guess the next thing I could try is to uninstall and then reinstall.
