Exactly one pair in Prolog list

Question

Does anybody know how I can go about determining/ensuring that there is exactly one duplicate element in a prolog list?

I am studying for a test.

Answer 1

one_duplicate(L) :-
    sort(L, Sorted),
    length(L, Len),
    length(Sorted, SortedLen),
    Len =:= SortedLen + 1.