[gdm-list] multiseat disable multiple logins

[guy duportail <guy linux-service be>]

I have a multiseat with ubuntu-gdm for 8 users.
I tried several configurations:
disable multiple logins for the same user: X is crashing upon login of two the 
same users.
With the doubleloginwarning=true there is indeed a warning,but the users can 
still login,but he is already logged in on another screen.
What is the perfect setup for disabling double loging of the same user.
A few months ago I did the trick but forgot it:on login of the same user there 
was just a message:user is already logged in.And the login screen stays.

guy