@Viva, I didn't forget our Back To The Future discussion. I've just been busy. I rewatched the beginning but I was rushed....anyway I didn't see the extra Marty. However I looked it up and have found some interesting articles alluding to the point you're making.
The Case Of The Two Martys
The Other Marty McFly Theorem
I know but I agree with the writer that it's probably a grip or someone from the set.
there IS that clock in Doc's house at the beginning that shows him dangling from the school tower...