Maybe I've read too much sci-fi dealing with time travel paradoxes, but:
Just before Marty McFly returns from the 50's, he decides to set his return for 10 minutes earlier than he left. He returns in time to see himself get in the DeLorian and go back to the 50's.
There are now two Marty McFly's.
The original one goes forward in his original time. The second one that just went back to the 50's will again return 10 minutes earlier than he left. He returns in time to see himself get in the DeLorian and go back to the 50's.
There are now THREE Marty McFly's.
Etc, etc, etc. What we end up with is an infinite Marty McFly generator.