As the `ContainsAll` is working with a match counter, it could return
a false positive when the `haystack` slice contains duplicate elements.
This can be checked with the included testing scenario, with
`haystack = [1, 1]` and `needles = [1, 2]`. Iterating over the haystack
to check for items to be present in needles will increase the match
counter to 2, even if `2` is not present in the first slice.
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