After posting a message, the forum's thread list page is displayed. If you browse from there then hit the browser's "Back" to see that listing, then reload it to get a freshened list, it's tantamount to resending the form for posting your message.
The forum software detects this and disallows the unintended duplicate posting, but there's a better way.
Right now the last step in gforum.cgi's "do=post_post" routine is to display the forum thread list. If instead, the last step were to print