DeleteDuplicates not deleting duplicates

Question

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.

Update:

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:

f[x__]:=Equal[x]
DeleteDuplicates[{5,5.},f]
(* {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.

Answer 1

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.

Answer 2

You need to use :

DeleteDuplicates[{5, 5.}, Equal[#1, #2] &]
(* {5} *)