[AChimp]

It's the same Marty, but a different version of him, from a separate point in time that happened to go back in time to a point where he already existed.

Imagine you going back in time to write this post, then returning to the present, then going back in time again but to some time slightly different (maybe a few seconds earlier or later). You'd be in the same situation as the movie and get to see yourself typing :O