;; check-duplicates : number nlon -> boolean
(define (check-duplicates n lst)
  (cond [(empty? lst) false]
        [(= (first lst) n) true] 
        [else (check-duplicates n (rest lst))]))

;; remove-duplicates : nlon -> nlon
(define (remove-duplicates lst)
  (cond [(empty? lst) empty]
        [(check-duplicates (first lst) lst) (remove-duplicates (rest lst))]
        [else (cons (first lst) (remove-duplicates (rest lst)))]))