This is not a mistake. The first translations (Ruby, Python, etc... 8 years ago) didn't test for duplicates. Duplicates were introduced by the Haskell translator (@bkaes) and gave many fiery discourses (see at the very bottom of this page). I never had enough time to modify that and don't think I will have. All I can do is to modify the description by replacing "Beware: r must be without duplicates." by "Beware: in some languages r must be without duplicates.". Hope this will satisvy you though not quite satisfying...

In the homepage.

Exactly.

Please use spoiler flag next time.