Why is everybody still talking about a time loop? Hasn't it been established by now that it's NOT a loop, not even if you look at the first movie independently? I think even the producers said so. And how could it be? Marty changed his past and returned to a different future with a different Marty, who then left for some unknown adventure. That second Marty could actually run into original Marty in 1955 unless Doc made other plans and set a different date. But we don't know, because it was never mentioned.
Besides, I guess not even Doc knows for sure how all this time-travel stuff really works, after all he only just made his first successful time jump. All the studying of time beforehand may have given him a good amount of knowledge to base theories on, but honestly... look at the movies and you'll see he's not very accurate in his predictions.
I think most of it is guesswork on his part. Educated guesses surely, but guesses nonetheless. So... long-winded story coming to a close... maybe he just sent the second Marty back in time to be sure. What if it WAS a loop? How could he be sure? How could he even know the past (original Marty's past) has in fact changed? He was definitely aware of the possibility as seen at the end of Part I, but he didn't know the original history well enough to be sure it changed drastically.
....now where was I? Oh yeah, it's NOT a loop!