Forms that can be accidentally, or maliciously submitted multiple times can cause havoc with your online application. Resubmits can happen for many reasons, mainly through page refreshing, back button navigation and multiple button clicks.
Fortunately there are a number of ways to prevent these actions from resubmitting a form.

I just finished watching an excellent presentation of the spry ajax framework by Ray Camden, an absolute must see for anyone exploring ajax.
In any case, during the presentation I noticed Ray using the Firebug firefox extension… I had never seen this before
and as soon as the presentation was over I went and installed it….