When Marty travels back to 1985 from 1955 at the end of the 1st BTTF movie (and meets his all new present), aren't there two Marties in that moment (the one that has just arrived and the one that was born thanks to his collaboration in making their parents to fall in love one with each other in 1955)? If not, why?
OK, now I realize of it. It has to do with this other question: What happens to the second Marty at the end of Back to the Future?
There are not two Marties in 1985 except during the moment in the shopping mall, when the newly arrived Marty sees the other Marty travelling back to the past, which closes the circle.