You can get up to one offer per
offer round. This means that you could hypothetically receive an offer for one course in December, accept it, then move your preferences around so that you’re in the running for an offer for another course in the January round. If you did receive a second place offer this could be from the same uni, yes.
However (and
A1 can correct me if I’m wrong) if you have UNSW medicine as a higher preference than medical science for the December round, then I’m inclined to believe you wouldn’t get the medical science offer in December because you would still be in the running for UNSW medicine (which wouldn’t have completed its selection process by December, obviously).
Hopefully that clears it up for you?