He should be called Felix II, and his successors of the same name should be numbered accordingly. The discrepancy was caused by the erroneous insertion in some lists of the name of St. Felix of Rome, Martyr. He was elected during the exile of St. Martin I, who endorsed him as pope. After St. Zachary died, a Roman priest named Stephen was elected but died before his consecration as bishop of Rome. His name is not included in all lists for this reason.
Confusion exists concerning the legitimacy of claims.
If the deposition of John was invalid, Leo was an antipope until after the end of Benedict's reign. If the deposition of John was valid, Leo was the legitimate pope and Benedict an antipope. If the triple removal of Benedict IX was not valid, Sylvester III, Gregory VI, and Clement II were antipopes. Elimination was made of the name of John XX in an effort to rectify the numerical designation of popes named John. The names of Marinus I and Marinus II were construed as Martin.