Classic question once again :)

Yes, it is possible to grab a users' IP but it's not 100% guaranteed that IP address belongs to a single user (as in proxies, firewalls, etc) or that the IP remains with the same user (as merlyn pointed out numerous times : AOL).

Possible solutions are Cookies... But again, no way of knowing for sure. Cookies can be rejected, cookie caches can be cleared, cookies CAN be forged, etc.

What about storing the last submitted form and checking if the newly submitted form is the same??

Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.