hi mentalman,
This is an annoying glitch which seems to occur when the server is running slow - it happened to a couple of people a few hours ago.
The best thing I can suggest at the moment is that when it occurs, check the thread you have posted on or the forum if it is a new thread, as you said, if you click submit again it tends to duplicate the post.
Having said which, a mod will usually be along shortly to delete the duplicate post for you. If doubtful as to whether it has been posted, you can always select your text and copy to clipboard, or just click back in your browser to return to your post which should still be there.
Dunc