;; duplicates?: list --> boolean
;; returns true if there are any duplicated elements 
;; in the given list, l.
(define (duplicates? l)
  (cond
    [(empty? l) false]
    [(cons? l) (or
                (duplicates?_help (rest l) (first l))
                (duplicates? (rest l)))]))

;;duplicates?_help: list any --> boolean
;; returns true if x equals any element in l
(define (duplicates?_help l x)
  (cond
    [(empty? l) false]
    [(cons? l) (cond
                 [(equal? x (first l)) true]
                 [else (duplicates?_help (rest l) x)])]))

(boolean=? false (duplicates? empty))
(boolean=? false (duplicates? (cons 'a empty)))
(boolean=? false (duplicates? (cons 1 (cons 2 empty))))
(boolean=? true (duplicates? (cons "hello" (cons "hello" empty))))
(boolean=? true (duplicates? (list 'a 2 "xyzzy" 2 6)))
(boolean=? true (duplicates? (list 3 'yahoo 55 55 55)))
(boolean=? false (duplicates? (list 'x 'y 'z)))