A plot hole in their time travels

So the movie establishes that whether they go to the same timeline as it would continue into their future or an alternate future when they go back into the past to spawn a new timeline, what happens is there is a second version of them there.

So when 1985 Marty and Jennifer went to 2015 there were two of each. It’s the same timeline they’d have ended up in had it just continued on without intervention.

So when Marty finally goes back to his 1985 there should already be another Marty there. A second Marty in 1985 alternate timeline who experienced everything and exists now.

But that is clearly not the case.

The 2nd movie clearly had 2 Martys instead of the same Marty taking over himself.

So when the 1955 Marty went back to 1985 there should be 2 Martys because that’s a diff timeline now where his parents are richer and Biff is cleaning their car.

Just like how when 1985 Marty went to 2015 there were 2 Martys.

So when he goes back there should still be 2 Martys in 1985.

This is a huge flaw in the otherwise spectacular movie.

Now I know the usual prowlers will chime in here saying it’s not but they’re wrong and ignored.

It’s clearly a plot hole now.