ok,
First of all its not the dog who becomes immune to the flea treatment, its the fleas.
Yes, in some areas of the country fleas are becoming immune to fipronil which is the active ingredient in frontline.
NEVER EVER use two kinds of flea treatment at the same time. It can make the dog very ill.
Finally did you treat the entire house including anywhere that your dog doesn't go, including toilets, bathrooms and wardrobes, AND the car and any other cats or dogs that you have or that your dog spends time with? For the environment (NOT PETS) I use RIP Flea which isn't cheap but goes a long way and works. Read the instructions carefully and be sure to follow them.
I would go back to your vet and get advice on how soon you can treat your dog again and what with, because, as I said, it can make the dog ill to keep piling on flea treatments.