Bug introduced in 9.0 and fixed in 10.0.0

It is not surprising that DeleteDuplicates[{5,5.}] returns {5,5.} because DeleteDuplicates uses SameQ by default, and SameQ[5,5.] is False.

However, Equal[5,5.] is True, but DeleteDuplicates[{5,5.},Equal] still returns {5,5.}.

It is interesting to note that Union works properly, in that Union[{5,5.},SameTest->Equal] returns {5}, as expected.

Is this a bug, or am I missing something? I'm using version 9.


I realize that using the function #1==#2& will get it working, thanks for pointing this out.

However, my question really is "Is this a bug, or am I missing something?".

To illustrate this further, consider the following:

(* {5} *)

In what capacity are f and Equal different? Or what is leading DeleteDuplicates to treat them differently? They both take an arbitrary number of inputs, which is what I originally thought was the problem.

  • 6
    $\begingroup$ I'm reporting this as a bug. $\endgroup$ Jan 23, 2013 at 17:24
  • 5
    $\begingroup$ But @Albert Retey, are you saying that you won't believe me in a comment, but you will in an answer? $\endgroup$ Jan 23, 2013 at 17:40
  • 6
    $\begingroup$ ... it might also be of interest that this behavior is new to version 9, version 7 and 8 give the "expected" result... $\endgroup$ Jan 23, 2013 at 17:41
  • 2
    $\begingroup$ (Sigh) @Albert Retey, I forgot your country of origin... $\endgroup$ Jan 23, 2013 at 17:44
  • 15
    $\begingroup$ Meta question: If this question ever gets a duplicate, will we be able to delete it? $\endgroup$ Jan 23, 2013 at 18:27

2 Answers 2


As @Albert Retey remarked in a comment, a second argument of (just) Equal leads to a special case handler that, to me, seems overly ambitious. I reported it as a bug.

  • $\begingroup$ +1 but... raking in rep for confirming bugs may be the wrong incentive, WRI-wise? :-P $\endgroup$
    – Yves Klett
    Jan 24, 2013 at 9:58
  • 2
    $\begingroup$ My strategy has been to fix bugs/implement features, remain silent for weeks/months, and then miss the rep uptick when the new version ships because sleep started to look better than beating out intrepid explorers for reputation. Maybe I should rethink that... :) $\endgroup$
    – John Fultz
    Jan 24, 2013 at 11:37
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    $\begingroup$ @Yves Klett I'm as confused as you. Meanwhile other responses take real work and get maybe 2 upvotes. $\endgroup$ Jan 24, 2013 at 15:13

You need to use :

DeleteDuplicates[{5, 5.}, Equal[#1, #2] &]
(* {5} *)
  • 1
    $\begingroup$ Something like Unevaluated@Identity@Equal will work a bit faster I think $\endgroup$
    – Rojo
    Jan 23, 2013 at 22:03

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