[gdm-list] double login

[duportail <guy duportail be>]

ubuntu 9.04 with gdm 2.18 with multidsiplay/multiseat setup:
i can login root twice on the same machine in gnome or kde.
But if I login the a user twice on the same machine the xserver crashes.
Gdm's doubleloginwarnig is ignored.
Why can root login twice without the xserver crashing and a user not.
Is there a way to prevent double logings with gdm?

regards,
guy