St. Pope is not "redundant" as much as it is "incorrect."  One may not wish to confuse, for example, Pope St. Alexander, with another St. Alexander.  A "rule" about dropping the title in place of "Saint" or vice versa works well enough if one is a close contemporary of the saint to whom reference is made (or if there is only one by such a name) but it is certainly not "redundant" or superfluous to have the two referants in every case.