When should two Cards have the same unique number?

i'm experimenting with tweaking CardFactory to be more efficient. don't worry, Chris H. i won't commit before the beta.
at first glance, it seems sensible that each Card be given a unique ID every time by the Card constructor. this makes great sense to me. and that's what happens. except that certain copy operations also copy the unique ID. that baffles me. can anyone explain why or when this is a good idea?
is it so the card's identity is still accurate even when a copy of it has been placed on the stack?
i think not handling this correctly is why my prior efforts with CardFactory failed. not that i want to resurrect that code really. i just think it's the reason.
at first glance, it seems sensible that each Card be given a unique ID every time by the Card constructor. this makes great sense to me. and that's what happens. except that certain copy operations also copy the unique ID. that baffles me. can anyone explain why or when this is a good idea?
is it so the card's identity is still accurate even when a copy of it has been placed on the stack?
i think not handling this correctly is why my prior efforts with CardFactory failed. not that i want to resurrect that code really. i just think it's the reason.