Your code is just an obfuscation of what the original post said didn't work

I'd say my code is clearer as it illustrates that grep is looking for elements that it has only seen once, IMHO at least. As for the question's premise being incorrect, I should've duly noted that their code was working but their question was 'wrong' (as I've done so earlier with the same (or seemingly so) AnonyMonk) but I guess tiredness got the better of me. You post, you learn ;)
HTH

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broquaint