The problem is the same as it is here with the chatterbox, if a page is refreshed or submitted twice, a double post will occur.

A very elegant solution was proposed for this problem by swiftone. The idea was to include with every page a unique ID (unique for 1 hour or so) and that the script would keep a list of which unique IDs had submitted a chat line. If a page was reloaded, that would show up as a double ID and the script could then determine what to do (serve the old results, process the new/old/resubmitted query etc.).