[O] How to remove duplicate entries?

[gylns]

Hi all,

In "2.5 Structure editing" of the manual of Org Mode, It says:
 "C-c ^     (org-sort-entries-or-items)
  ... With two C-u C-u prefixes, duplicate entries will also be removed."

I had tried this but it seems doesn't work.

* test
** test
** test

I pressed twice C-u and then C-c ^ , sorted by alpha, but no entries be 
removed.

Any one can tell what's wrong about my understanding?


Cheers,
gylns