bug#4657: 23.1.50; duplicate entries in file-name-history from read-file-name

In a fresh emacs -q load and execute the command foo in order to
read the file name ~/foo/bar

(defun foo ()
  (interactive)
  (read-file-name "File: (default ~/foo/bar) " "~/foo/" "~/foo/bar")
  (message "%s" file-name-history))

If I explicitly enter ~/foo/bar in the minibuffer, multiple
executions of this command add this file to file-name-history only
once. If I am lazy and just use the default value, each execution
will add one more entry ~/foo/bar to file-name-history. I would
expect that multiple equal entries in succession should always be
avoided in file-name-history.



In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, GTK+ Version 2.12.9)
 of 2009-08-03 on foo

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: C
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: en_GB.utf8
  value of $LANG: en_US.ISO-8859-15
  value of $XMODIFIERS: nil
  locale-coding-system: iso-latin-9-unix
  default-enable-multibyte-characters: t

Major mode: Lisp Interaction