The vaccine is pretty safe, and there should be no side effects for your cat. One of my kittens somehow grew an abscess on his neck (probably a roughhousing incident with the other kits) and since abscesses usually appear secondary to bite wounds my vet insisted upon re-vaccinating him, even though he'd just been vaccinated two months before.

My first shelter job was at a huge shelter that processed dozens of animals every week, plus did vax clinics, etc. Double vaccinating, while not something we encouraged, certainly did happen now and then. As far as I know nobody ever died of it. I think the margin for error in most vaccines is actually pretty high - in other words you'd need a lot more than one double-dosing incident to harm your cat. (Unless he has some untoward reaction/allergy to the vaccine, which is an entirely different story.)

Hope that helps!