I think that bitcoin wallet would not allow anybody to have a collision with an existing address that would be not a problem of every users of bitcoin,
well if these kind of problem would occur in bitcoin, people would just make their new account and hope they can get that has a bitcoin on it, There would be an uproar in the bitcoin community that certain bitcoin would just ended up to another person
The probability of that happening can be close to nothing. I am sure there is a way to check and monitor so that in case it can happen then it can be remedied fast.