The question isn't whether or not it will happen. It will almost certainly never happen. The question is, "What if it does?"
In a technical sense nothing bad would happen. It would be exactly the same if you had the same private key on two devices, which is something I do all the time, for example. Each owner of the account could spend any amount they wanted to.
In a social sense, I think all hell would break loose. There's so much certainty that it will never happen, that if it does people's faith in the system would lessen. But since it would only effective at most two people, one or the other of them would simply drain the account and then everything returned to normal.
Most people would probably assume that one user just stole the other user's key.