Does a list contain duplicate references?

list-uniq.ss

(define (list-uniq l)
  (define t (make-hasheq))
  (define (check l)
    (or (empty? l)
        (not (cons? l))
        (and (cons? l)
             (not (hash-ref t l #f))
             (begin
               (hash-set! t l #t)
               (check/list l)))))
  (define (check/list l)
    (or (empty? l)
        (and (check (car l))
             (check/list (cdr l)))))
  (check l))

(define a '(1 (2 (3 4)) 5))
(list-uniq a) ;#t
(list-uniq (list a a)) ;#f