I opened a PR. By default, it raises an exception if there's an invalid identifier; there's a keyword argument raise_on_invalid to control that.

The implementation I have adds them to a set first, which means the order is not guaranteed. I'm of two minds about this: if there's a big template, you want to gather the identifiers in a set so uniqueness is checked immediately and O(1) and without duplication. On the other hand, if templates are never very big, you could build a list (in order) and check it, O(N) style, or build a duplicate list and set in parallel. Or build a big list and check it at the end.

I kind of think ordering doesn't matter? What would someone do with that information?