The Apollo 11 backup crew was the prime crew for Apollo 13.
Crew scheduling was somewhat messy, but generally the backup crew for a mission were the prime crew for the next+2 mission. (13 and 14 crews were swapped for reasons).
Let’s not forget that these were all very complicated missions and not duplicative. It would be nearly impossible to train for two at once, and do it well. Too disfocussing.
Well, you know, except for descent, touchdown, exiting the lunar lander, performing some exploration, gathering moon rocks, re-entering the lunar lander, take-off, and lunar orbit insertion.
Crew scheduling was somewhat messy, but generally the backup crew for a mission were the prime crew for the next+2 mission. (13 and 14 crews were swapped for reasons).