If someone double-clicks (or clicks it 8 times) your submit button, your script could be run more than once. Try disabling the submit button on click (using Javascript).
Another, probably unnecessary, solution would be to lock your form script (e.g. using semaphores), create a database entry with the provided info, then check to see if you've sent the same info recently (e.g. within the last five minutes), and don't sent it again if you have.
I'm sure the Javscript disable functionality will suffice :)
Or, if you want to find out what's really going on, you could just create a log file that logs "sent [some distinct piece of data that might tell these posts apart] at [timestamp]". If you see a couple of these, with the same [distinct info] entry and really close timestamp, then your problem is multiple clicks to the submit button.