bug#18181: bug in display-buffer-below-selected

[Trevor Jim]

Hello.  I am seeing what I think is a bug in
display-buffer-below-selected in recent versions of emacs-24.3.

Namely, if you execute display-buffer-below-selected when the buffer
is already in a window below selected, you will get *two* windows
showing the buffer.  A simple test is to run

    (let ((buffer (get-buffer-create "test-buffer")))
      (display-buffer-below-selected buffer nil)
      (display-buffer-below-selected buffer nil))

in *scratch*.

This behavior seems to have been introduced in git commit
    6a5d9e9a3247566bf69575444e2a1f75f5b3e021
in the emacs-24 branch of git://git.savannah.gnu.org/emacs.git

-T