Jake Mcarthur wrote:
Heh, after a couple more seconds of thought, reversing the two composed functions fixes it:

    nub . permutations

Of course, neither my previous nonsolution nor this solution are efficient for long lists, but I think it serves as a decent reference implementation at least.

Consider the following list:

 [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2]

Somebody just pointed out to me that this list has 87,178,291,200 permutations, of which 14 are unique. O_O

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