Couldn't the same be said about the multiple times they went back in time to get the friend to ace his Chem test? They should have run into themselves traveling back in time multiple times, no?
Exacty, you can only re-do the past if your consciousness travels back ("Buttefly Effect" movies). If you physically travel back, there should be a physical version of you from each trip at the same time.
At least one science fiction novel explained this away by saying that the re-do self will talk the first-trip self out of doing whatever it was he wants to undo. This movie makes that impossible, obviously.
One more possibility, however, that may fit within (what passes for) this movie's continuity: if you travel back before your earlier trip and somehow prevent your earlier self from making the trip, without him seeing you, you could replace him during the earlier trip ("Primer").
There's nothing in this film to indicate that level of consideration, however. The big inconsistency at the end confirms this (i.e., the fact that the protagonist vanishes but his camera does not, even though they both from the same timeline)