most of my duplicates are from wanting games on the newer systems even though i have them on the original console (which is with my parents)
super mario bros. (NES, WiiVC)
kirby's adventure (NES, WiiVC)
castlevania III (NES, WiiVC)
super mario bros. 3 (NES, GBA)
donkey kong country (SNES, GBA)
super mario world (SNES, GBA)
gta: vice city (PS2x2) - i let someone borrow it and they scratched the disk to hell. got another one, and i still don't care to finish it
soul calibur 2 or 3 (PS2x2) - some scratched it on accident and i got another copy
starcraft - lost the disk at some point and got another copy.
i also have almost all of the games in both the mega man anniversary collection and mega man x collection on their original formats, i think i'm just missing x2 and x3 on SNES.
i'd rather just have one permanent copy though than buy a digital one.