I ran some tests and this happens in the following browsers:
Safari (OSX Jaguar)
Opera (latest)
FireFox 1 & 2
MSIE 5.5 and up
The problem is probably to do with the way your session is saved. Navigating away from the forums before closing the browser seems to fix it. Also these blasted forums have issues if you open more than one window or tab.
EG: if you are replying to a post, do not click a link on these forums in ANY open window or tab ohterwise it gets confused as to your last transaction and you will get a duplicate post error. If only the post ID was assigned on posting rather than on opening the reply screen.