As far as Iknow, the only reason pups need two vacs is that the mothers anti-bodies are likely to reduce the effect of the vaccine prior to 12 weeks, so two are needed to ensure the pup becomes immune. Thsi is also why vets incresingly suggest the second vacc is given at or after 12 weeks. I am pretty sure that if you vaccinate at 16 weeks or later, only one vacc is needed.

If the booster is ever over a year from the last jab, you still only need one injection, no need to start the cours eagain. Yet still, some vets insist you need both injections if your booster is over.