Just an idea.
Before posting a message, the system would verify if the same user have not already posted the same message in other section, if so, the system would show a message telling the user that duplicate messagens are not allowed.
Sorry the poor english. [originally posted on jforum.net by Anonymous]
The phpBB's quite regular-showing-itself bug (at least, in earlier versions) was that user didn't get any feedback about message being accepted (like null server responce or some funky error message being displayed instead), so user pressed "Reload" in browser again and again - and that made post to be added several times. Not the case of JForum, at least should not be [originally posted on jforum.net by Daniil]