bug#12275: 23.4; "C-c C-b" message-insert-newsgroups insert duplicated entries.

[Oleksandr Gavenko]

In GNU Emacs 23.4.1 (i486-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2012-04-08 on biber, modified by Debian

When I replay to message in buffer with message-mode and place "Newsgroups"
header by "C-c C-b" next call of message-insert-newsgroups insert same entries
as many as I call discussed function:

  Newsgroups: 
gmane.emacs.help,gmane.emacs.help,gmane.emacs.help,gmane.emacs.help

When I try call "C-c C-t" message-insert-to command many times I would be
warned like:

  already have `Oleksandr Gavenko <gavenkoa@gmail.com>, 685815@bugs.debian.org' 
in `To'

I think that message-insert-newsgroups must also warn about duplication and
prevent them.

-- 
Best regards!