Help please!!!

Rating: 9/10 (15 September 2004)

Alright can someone explain this concept or paradox to me- This comes from near the end of the 1st movie. Marty has decided to set his return time earlier than the time he originally departed. This is so that he can warn the Doctor of his coming death. The thing is when he does this and gets to the car park because he's late, he sees the attack taking place from a 3rd person view and he sees himself get into the time machine and drive into 1955. If Marty, was able to get there in time to warn the Doctor and they managed to avoid the attack, what would have happened in 1985 because we would have had 2 Marty(s), from 1985, since Marty who jumped in the machine would not have had to go to 1955?