The fundamental principle of the universe that Physics still doesn't explain: no two stones are identical, no two leaves are identical, no two cats are identical, no two hands are identical, ... no two planets are identical, no two stars are identical.

There are billions of stars in the sky but there is no geometry. And, if you look close enough, they are all different. Ditto for the rocks of this planet and any other planet, and ditto for everything that nature does. All fundamentally different from each other.

It's a fundamental law of the universe, but it does not show up in the formulas of Physics: how deterministic laws can produce a natural world that never repeats itself.

This is not about randomness. There are very deterministic causes that lead to the shape of a rock or of a planet; but those causes, somehow, always lead to something unique, to something that never was before.

That's why a copy of you doesn't exist and never will: no two brains are identical.