Merry Christmas wrote:
On that side of the screen: One could infer that a universe-annihilating paradox would occur if two DeLoreans were in the same place at the same time.
But, when it is used to travel to 1955 the second time, the DeLorean is older and therefor not the exact same car.
Doc did modify it.
Plus, we already know from earlier in that film that meeting your future/past self doesn't create a paradox. Jennifer/Old Jennifer simply faint from the shock of meeting each other. Doc is able to hold a conversation with past-Doc with no problems. Actually, to go back even further, Marty sees past Marty at the end of the first Back to the Future when he returns to Lone Pines Mall (although they don't interact).