he's played center since David Robinson retired.
In 1997-2003, Robinson was clearly the center. Backup was Will Perdue, Kevin Willis.
In 2003-2006, Nesterovic was clearly the center. Backups were Kevin Willis, Nazr Mohammed.
Then it was Obeeto from 06-09.
Duncan has spent the majority of his years as a PF, won most of his rings as a PF, and likes to be called a PF. As he's gotten older, he guards more centers and stuff now but front court positions are changing anyway where PFs play C and vice versa.
Just because Lebron and Melo spend a lot of time at PF the last couple of years DIESNT mean they're aren't still SFs.