and the vet says since its overdue, she will need a double dose.

my cat is 4 months old, she was given fvrcp on 2/25/12 and was due on 3/20/12 but i didnt have money on that day to give her the shot. since it was a free check up given by the shelter i didnt think id need to give her any necessary shots besides the rabies shot, which she got. now the vet says since im late ill have to pay to give her 2 doses of FVRCP. he also says she needs a respiratory virus vax. im not sure if she really needs these shots.

shes an indoor cat, but im scared shell get out one day and get sick. any help would be appreciated.

Some of the vaccines need a second dose to be effective, so your vet is right. I've honestly never heard of a double dose, as I've always been able to get them their second round of shots on time.

Kittens always need a booster after 16 weeks or they won't develop long-lasting immunity. Anything given before that may not fully "take" due to maternal antibodies floating around. And some vaccine do need at least 2 doses to produce full immunity. I would get her an FVCRP booster.

Do you know what the vet means by "respiratory virus vaccine"? I'm not aware of any other vaccine for cats.

The double dose thing is wrong. And against label instructions--possibly dangerous. I think I'd switch vets.