Heh. Sounds like a disease. "Remember to wash your hands before posting or you might get DPS..."
I think Rarebit stole what I said in the admin forums Anywho they know about it and should be working on a fix...
It happens if you have opened another forum post in a separate window while you have a post, reply, or PM form open. So if you say hit "Reply" on a post, then open a different post in a separate window, and try submitting the reply in the first window, you will get Duplicate Post Submission.