>>Also, there are cases- some reference tables for example- where there are no natural keys and enforcing natural keys would be weird.
>
>Theoretically, I'm not sure that's true. There has to be "something" about each row (even if it's a description) that logically constrains the row as unique.
>
>(Or maybe this is just a matter of semantics)

Yes. You're right, people shouldn't duplicate anything.
I've seen business situations where a reference entry was duplicated with no harm done, but that was luck.
It's potentially harmful.