Revealing mistake: Doc and Marty hide in the alleyway while the flying police car takes 1985-Jen home. Problem is, Doc stares in awe at the sky before the police car is in view.

Back to the Future Part II (1989)
21 revealing mistakes
Directed by: Robert Zemeckis
Starring: Christopher Lloyd, Elizabeth Shue, Lea Thompson, Michael J. Fox, Thomas F. Wilson

Revealing mistake: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible. (01:30:30)
Biff: Go ahead, kid. Jump. A suicide will be nice and neat.
Marty: What if I don't?
Biff: [raising gun] Lead poisoning.
Marty: What about the police, Biff? They're gonna match up the bullet with that gun.
Biff: Kid, I own the police. Besides, they couldn't match up the bullet that killed your old man.
Marty: You son of a...
[Biff pulls back on the gun's hammer.]
Biff: Suppose it's poetic justice. Two McFlys with the same gun.
Question: In the alternate 1985, there is an alternate Biff, Lorraine etc. Shouldn't there also be an alternate Marty and Doc?
Answer: Doc would most likely not have been seen by anyone, as the time he spent in the alternate 1985 was primarily inside the DeLorean, at a boarded-up library, graveyard, and his lab (and all at night too) so most likely not spotted by the public.
Even if someone had seen Doc, it could've been dismissed as someone who looks like him. Even if they did report his escape, someone would either call or go to the asylum and verify Doc was still there.
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Answer: Yep, and there is, but they're both elsewhere. Doc's been committed to an asylum somewhere. When Marty first meets the alternate Biff, Biff tells him that he's supposed to be in Switzerland at boarding school - that's where the alternate Marty is.
Tailkinker
Wouldn't someone probably see Doc and report that he escaped from the asylum?
Maybe, but no way to be sure, and they're not around long enough for that to be an issue anyway.
Jon Sandys