If true, that was Gregory XII who was one of three popes. He resigned essentially so there could be one pope. (so sayeth google)
Not to question google, but, although there were three men claiming to be the pope, only one would be the true pope. The other two would be regarded by Catholics as "anti popes."
In every era, there are oddballs who claim to be pope. That wasn't the case in the 1400s, where the true papal succession was controversial.