General Discussion

When Marty goes back after the dance, he's back in his own time.
And, your other 2 examples violate the same physical principles.
The same matter, with the same quantum probability distribution in 2 places at once.
The starting point doesn't matter. It requires the creation of matter that doesn't exist.
We can forget about the quantum probabilities of two lumps of matter being identical. That just makes it worse, but isn't needed beyond the lack of matter available to have 2 Martys.