if you are unsure they can do a serology test to see what you are immune against. this is a simple blood test results back within 24 hours most times.

but generally no you cant "overdose" but with soem vaccines it is possible to have a reaction to it usually only locally like a swallon red bump at teh injection site if you have only recently had it or your already immune, as the immune response starts immediately as it knows it is a bad thing and tries to get rid of it.

which vaccines are you loooking at getting in particular if you dont mind me asking and i can give you some more specific answers about it