S9 LIB  (duplicates list)        ==>  list
        (dupp procedure^2 list)  ==>  list
        (dupq list)              ==>  list
        (dupv list)              ==>  list

Return a list of duplicates contained in LIST.

DUPP uses the predicate PROCEDURE^2 to check if two members of
LIST are duplicates.

(Duplicates x)  is equal to  (dupp equal? x)
(Dupv x)        is equal to  (dupp eqv? x)
(Dupq x)        is equal to  (dupp eq? x)

When a duplicate occurs multiple times in a given list, it will
be contained only once in the results of these procedures.

(dupp = '(1 2 3 1 2))        ==>  (1 2)
(duplicates '((1) (2) (1)))  ==>  ((1))
(dupv '(#\a #\b #\a #\c))    ==>  (#\a)
(dupq '(a b c d a c e f c))  ==>  (a c)
