In the link below, Dr. Peter Chojnowski presents a quote from St. Robert Bellarmine showing how popes can err in judgment about the legitimacy of popes who came before them. This should make those think twice who today say that we need to wait for a future pope to make a judgment about who the true pope is currently. No. Never in the history of the Church has the resolution of who was the true pope when there was more than one claimant been decided by a future pope. It was always decided during the lifetime of the claimants. So, too, must the identity of the current true pope be resolved in our own time. For anybody who has eyes to see and ears to hear, they can fairly easily come to know that it is Benedict XVI.
The following link contains a video in which Fr. Paul Kramer explains why the identity of the current true pope must be resolved today.