Making this change would cut down on duplicate banrequests (since we sometimes don't know if it's one spammer or several) without giving away the posters IP.

If giving away the banrequest reasons is too much information (I don't see how it could be) it could just say "X requests already filed"

This wouldn't do anything for the [B] button in the extension, but that could be modified so that when it tries to submit the ban request, it checks if an existing one exists and presents a "There exist X banrequests for this IP already. Still submit? [yes] [no]"