Yeah, I had a look at python solutions and if I ignore the solutions that trivialize the kata with builtins, incorrect solutions are common.

It's a beginner kata, it should catch beginner mistakes, and this task makes it easy to create a slightly wrong solution that naive random tests don't catch.

I suggest adding these 75 exhaustive size-4 fixed tests to all languages (gotta love how each language has its own code):

import itertools

def rename(xs):
    # my implementation is probably smooth-brained
    keys = itertools.count(1)
    known = {}
    out = []
    for x, i in sorted((x, i) for i, x in enumerate(xs)):
        if x not in known:
            known[x] = next(keys)
        out.append((known[x], i))
    return tuple(x for (x, i) in sorted(out, key=lambda xi: xi[1]))

size = 4
xs = list(map(list, set(itertools.chain(*map(itertools.permutations, set(map(rename, itertools.combinations_with_replacement(range(1, size+1), size))))))))
print(xs)
print(len(xs))

[[1, 1, 1, 1], [1, 1, 1, 2], [1, 1, 2, 1], [1, 1, 2, 2],
 [1, 1, 2, 3], [1, 1, 3, 2], [1, 2, 1, 1], [1, 2, 1, 2],
 [1, 2, 1, 3], [1, 2, 2, 1], [1, 2, 2, 2], [1, 2, 2, 3],
 [1, 2, 3, 1], [1, 2, 3, 2], [1, 2, 3, 3], [1, 2, 3, 4],
 [1, 2, 4, 3], [1, 3, 1, 2], [1, 3, 2, 1], [1, 3, 2, 2],
 [1, 3, 2, 3], [1, 3, 2, 4], [1, 3, 3, 2], [1, 3, 4, 2],
 [1, 4, 2, 3], [1, 4, 3, 2], [2, 1, 1, 1], [2, 1, 1, 2],
 [2, 1, 1, 3], [2, 1, 2, 1], [2, 1, 2, 2], [2, 1, 2, 3],
 [2, 1, 3, 1], [2, 1, 3, 2], [2, 1, 3, 3], [2, 1, 3, 4],
 [2, 1, 4, 3], [2, 2, 1, 1], [2, 2, 1, 2], [2, 2, 1, 3],
 [2, 2, 2, 1], [2, 2, 3, 1], [2, 3, 1, 1], [2, 3, 1, 2],
 [2, 3, 1, 3], [2, 3, 1, 4], [2, 3, 2, 1], [2, 3, 3, 1],
 [2, 3, 4, 1], [2, 4, 1, 3], [2, 4, 3, 1], [3, 1, 1, 2],
 [3, 1, 2, 1], [3, 1, 2, 2], [3, 1, 2, 3], [3, 1, 2, 4],
 [3, 1, 3, 2], [3, 1, 4, 2], [3, 2, 1, 1], [3, 2, 1, 2],
 [3, 2, 1, 3], [3, 2, 1, 4], [3, 2, 2, 1], [3, 2, 3, 1],
 [3, 2, 4, 1], [3, 3, 1, 2], [3, 3, 2, 1], [3, 4, 1, 2],
 [3, 4, 2, 1], [4, 1, 2, 3], [4, 1, 3, 2], [4, 2, 1, 3],
 [4, 2, 3, 1], [4, 3, 1, 2], [4, 3, 2, 1]]

(maybe they need to be renamed again so the answer isn't always either 2 or 3)

size-5 lists might have some limited value too, there are 541 of those though.

This test was really well done. Gave me a good mental workout and really made me have to think about how to get the logic to work.

Found this enjoyable. Sneakily trickier than expected.

Found this really good. Quite challening but not too challenging so I'd give up. Kept me going to find the answer. Nice one

I was same

I think this was my favourite test yet

Yes - I see that now. Don't know what way I was looking at it last night. Thanks

No, the example is fine, 1 + 4 + 4 = 9, not 5.

The example in the description was wrong; should equal 5 not 9

Good challenge. Thought my brain was going to explode.