So, how is Marty going to warn doc about his murder if he shows up 10 minutes early and the other marty is standing right there??
AND...why would Doc set up the same thing to happen with the Lybians? In the present of that time line Marty returns to, Doc already knows about the murdering and in that present it's clearly the future of those past events they've changed time with...his parents have changed, and it's a alternative reality...
So why would Doc have Marty go back to 1955 again? Wouldn't that create another alternative time line or something??