Until today the word "duplicate" was clear to me, or so I thought, but I have just been confronted with the following situation: there is a question A from 2011 (with an accepted answer) and one other, B, from 2015, that asks the same thing (again, with an accepted answer). User U marks question A as a duplicate (!), starting the voting process, claiming that the usual meaning of "duplicate" should not matter and that the question with less (or poorer) answers should be considered the duplicate of the other and thus closed, irrespective of their chronological order. Is this the correct decision?
A: What's the name for the property of a function $f$ that means $f(f(x))=x$?