Elisp: how to find list duplicates

Question

I am using this to find list duplicates:

(defun have-dups (x)
  (let ((dups (copy-tree x)))
    (if (eq (length (delete-dups dups)) (length x))
    nil
      t)))

(have-dups (list 1 2 3 3)) ;=> t
(have-dups (list 1 2 3))   ;=> nil 

Considering the overhead of copy-tree and delete-dups, probably there is a better way.

Answer 1

Use a hash table, as soon as you find an element which already exists in the hash table, you know you have duplicates:

(defun has-dup (list)
  (block nil
    (let ((hash (make-hash-table :test 'eql)))
      (map ()
           (lambda (item)
             (if (gethash item hash)
                 (return t)
               (setf (gethash item hash) t)))
           list))))