While Unique<T> is a lang item and has special SB treatment, it still can be viewed as just a pointer. And no one can, or should, prevent unsafe code from making the same invariant, and your code will allow people to break that.
Copying values is such a fundamental operation that unsafe code relies on invariants regarding it a lot; that is why we have Copy. Copying things that are not Copy, that is, were never meant to be copied and moreover, many times they were meant to not be copied, is going to break a lot of things. If you want a trait for "can be safely duplicated", well, it has the name Copy.
Now there can (though I doubt that, since Copy is that essential) a type that is mistakenly not Copy and you cannot fix it for some reason, make a newtype and unsafely copy it, that's fine (assuming we suppose that's fine, see the above UCG issue). But doing that generically is going to hurt you.