What happens if the same vaccine is given twice to a puppy?

like if the pup is already vaccined against parvo and then a vaccine against parvo is given again (when the owner didnt know it was already vaccinated against parvo)is it harmful ? what happens?

Depends on the interval between vaccination. All puppies have to have at least 2 sets of shots (in the UK) but not days apart. If this has happened, you do need to speak to your vet. Vaccination is done to stimulate a response from the immune system, and logic suggests, to me, that if the immune system has already been kicked, doing it again immediately won't take, but it may make the puppy sick. Not sure about that - so ask your vet.ps Once the puppy sets are done, these days it's recommended that you booster in another 12 months, and then not again for 3 years apart from the vaccination booster for Leptospirosis which it 's considered needs to be done annually.

have to ask vet that knows dog and vaccine

It won't do anything. If you live in the US, the minimal requirement is 3 sets of vaccines, so if the puppy hasn't had all 3 sets, you need to schedule a vet appt to get the puppy vaccinated. Generally the vaccines are 2-3 weeks apart. However, if it's already had all 3 sets and you gave it a 4th set, it won't hurt it. Some people give 4 sets anyway (like me, who's worked in vet clinics half my life and constantly exposed to parvo; not taking any chances with my dogs) so it's not a big deal, and some vets prefer to give 4 sets anyway.