Revert "Delete duplicates from collection before completion"

This reverts commit 1b74a85fe6420e02b92a309bdd049f12241aacc0.

The call to delete-dups resulted in significant performance
defradation on large collections.

ido-ubiquitous.el

@@ -542,8 +542,7 @@ completion for them."
          ;; avoid unnecessary work if ido isn't going to used.
          (__ignore ;; (Return value doesn't matter).
           (when (and ido-allowed collection-ok)
-            (setq collection
-                  (delete-dups (all-completions "" collection predicate))
+            (setq collection (all-completions "" collection predicate)
                   ;; Don't need this any more
                   predicate nil)))
          (colection-ok