You should add a test for a rather large list, so that solutions that are not tail-recursive produce stack overflows.

This is not tail recursive, and as such is not optimized well. If the list is long, you'd end up with a stack overflow.

That's still a fragment an is in no way verifiable to check for. The slash is only a convention. There is nothing preventing a page from doing navigation from index.html#pathgoeshere (and several of my pages does this).

Test for links with multiple #-signs.

This would fail if your url had multiple '#' (totally legal as far as I know).

The description never specified that I should return false if there was no duplicates, and failed me for returning undefined.