EWTN Catholic Q&A
Fake Pope
Question from Angel Azor on 04-09-2005:

Is it true that there is a possibility that there could be two popes- the true pope & the antipope? (or uncanonically elected pope)? How can we know who is the true pope if that happens?

Answer by Colin B. Donovan, STL on 04-12-2005:

Yes, it is a possibility. In every case it has occured because a group of the cardinals, not seeing their candidate elected, went away and elected their own guy. These "popes," elected outside the law by a minority of the Cardinals, have been styled antipopes. We are unlikely to see something like that in our times. Unless a substantial number of the Cardinals come out of the Conclave raising questions of its validity, we can be certain that the Pope is the Pope.