The Other Marty
When Marty returns to Lone Pine Mall and sees the interaction with the terrorists unfold, he watches himself jump into the Delorean and go back in time.
But that Marty is not the same Marty as him. That Marty grew up with parents who were well-adjusted. So when that Marty goes back to 1955, what is he going to do? Unless he does the same as the original Marty did, he's going to return to a 1985 where his parents are weird.
And if his parents constantly told him about the Under the Sea dance, like they did with the original Marty, they would tell him about the weird guy who showed up for a week and changed their lives. (Somehow not recognizing that their son looked exactly like him.) Would the second Marty then realize he was that weird guy?
And the first Marty is going to have completely different family memories than his siblings and parents. Like, he'll say, "Remember that time Dad did that crazy thing?" And his sister and brother will be like, "What? When did that happen?" Or his sister or brother might say, "Remember that vacation we took to Hawaii?", and Marty would respond, "What? We could never afford that."