Seriously WTF. Why wouldn't Emmett Brown say to Marty 30 years later: "Sup man, you were in the past, you actually gave me the idea for the time machine and saved my life with that letter"
And why wouldn't Lorraine think when her child had grown up like: "WTF this guy looks exactly like Marty, the guy I liked 30 years ago and suddenly disappeared."
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Why do you think he has the bullet proof vest on when the Libyans shoot at him? Pretty sure he knows that Marty in the present is the Marty from the past, but he can't let him know until he actually goes back in time, otherwise the time line would be screwed up. I wouldn't be surprised if that was the reason that Emmet Brown had Marty working for him in the first place, a causal loop.