Correct: guys' night out (guy + s + apostrophe)
Incorrect: guy's night out (implies only one guy)
Duchesses night out. (two or more)
Duchess's night out (single)
Does Duchesses need the apostrophe also ? Doesn't ending in es imply more than one in any case or does it need:
Duchesses' night out.
It has to be duchesses' night out.
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