Loop of Marties in Back to the Future 1

There are 2 different timelines at play here. Let's call Marty1 the Marty that we know and love. He goes to the past and creates timeline 2 and changes a bunch of things which are now part of a new timeline. People from his timeline now are not the same and may not even exist. Which is why the people in the picture are disappearing. Luckily Marty1 doesn't affect things so badly that they are wiped from existence. When Marty goes back to 10min before he leaves. He is in the new timeline. The Marty he sees is Marty2. He can go down.and stop him.from going all.he wants. It won't affect him. They are not the same person. This is evident because Marty2 grew up in a better home with successful parents.

It's a plot hole, but one you shouldn't look too deeply into.

There is no Marty1 & Marty2, there is a single Marty. It's convenient, though, to use them as placeholders for which timeline we're talking about.

What you think of as Marty1, who comes back 10 minutes early to see his former self as Marty2 simply cannot happen [or might be extremely unlikely], because Marty2 wouldn't have needed to make the trip, or may not have even met the Doc at all. His entire life, background and upbringing would have been different.

This, however is beside the point.

What you need to do is suspend disbelief, or the plot fails.
It doesn't matter whether there's a paradox, or two independent time-lines with apparently the same plot-line happening in each.

At the moment this temporary duality exists, neither Marty1 nor the audience is aware that this current timeline is not the same as the one he left.
It's needed to round out some hanging points left over from the 'first time' Marty1 leaves. Those are dealt with, and then we move on, not examining too closely.
By the time he gets home, he realises there have been changes, for the better.
Surprise, cheers, applause. The audience goes home, satisfied - and not incidentally excited that they know there's going to be a sequel.

So, Marty1 sees Marty1, earlier in time. He does not see Marty2. There is no Marty2. The Marty who returns home never goes back to 1955 again. There is no loop.

Marty that traveled to the past arrives back in his originating time to see himself time travel, but that Marty doesn't travel again, he wakes up the next day at the end of Back to the Future. He is not the Marty in the Delereon dissappearing to 1955 he is the Marty watching.

As you say, Marty - let;s call him Marty1 - returns 10 minutes before he left, just in time to see all the events play out as from the start of the movie.

However, despite the fact that what he sees matches the start of the movie, what we soon find out is that the timeline that he has returned to is a very different timeline from the one that he left. In this new timeline, his father George is a successful author, who presumably never returned to his earlier peeping-tom activities.

So what our original Marty1 saw, was that this new timeline's Marty2 setting off on the adventure - NOT himself. That new timeline's Marty2 goes back to the new timeline's 1955 and presumably changes things there. This will not be the same 1955 timeline that we had just witnessed Marty1 visiting in the movie - each timeline can only have one deLorean receiving the lightening strike for example.

So Marty2 changes something in his 1955 timeline, so changes his timeline to something else, and returns to that. Maybe there's an infinite number of Martys proceeding in sucession.

Or maybe Marty2, the successful author's son, changed the timeline so George is no longer successful, and therefore Marty2 returns to Marty1's McFly family that we saw at the start of the movie.

Boy, is he in for a shock !