There can only be one real Pope at a time; this has been true ever since Jesus gave St.Peter the keys to Heaven and Earth.
However, during various points in The Middle Ages, there was more than one people claiming to be Pope of The Catholic Church. This weakened The Church as a whole, at one point creating a Western Schism (an event which almost split The Church in three, and which lead to the disastrous Protestant Reformation).
While it is easy to tell the Popes from the Anti-Popes in hindsight, it is less easy to do when it is actually happening. When in a situation similar to The Western Schism, how would a Good Catholic go about determining which Pope is real and which Pope is false?