LOL
Well, what do you want? you want to stop people from submitting the same thing twice.
So when should you refuse the submition? when the submitted data is the same as what he submitted before.
So how do you stop them from submiting twice? by checking wether what they submit has already been submitted. If it has already been submitted, you refuse the new submition.
A forum, a FAQ, email notification, what else do you need?