Finding duplicates

find-duplicates.rkt

#lang racket


(define (find-duplicates elts)
  (define ht (make-hash))
  (for/fold ([dups '()]) ([x (in-list elts)])
    (hash-set! ht x (add1 (hash-ref ht x 0)))
    (cond
      [(= (hash-ref ht x) 2)
       (cons x dups)]
      [else
       dups])))

(find-duplicates '(1 2 2 3 1 4 5 4 4 4))