One has a collar (the one in your pic) and the other is just a plain spacer with no collar. The plain spacer is narrower than the collared one. As Ceph said, the "missing" spacer will often stick to the wheel so check to see if it is still there. If not, then I would suggest crawling around on your garge floor on your hands and knees.
yea thier important i accidently for got the left side one on the rear tire and mr crush drive rubbed against the swing arm for like a day and now its scratched to **** i noticed when my bike didnt handle right