I don't have any duplicates as of yet. If I did...I'd probably sell them eventually. I'm kinda in a mode to collect ponies and let them accrue some value first. I've bought some MIB G4s from stores just for that reason.
Honestly, if it was an old childhood pony versus a duplicate in much nicer condition, I'd sell the nicer one. The old one is rich with memories, while the nicer one is worth more to serious collectors, so win-win XP