Since it is quite annoying, has a bug, and is not wanted when using a kiosk configuration alongside normal logins1, i wanted to disable or at least prevent the warning "You are already logged in".

Google did not spit out a suitable answer (at least with my normal Google-Fu), so searching through the gdm files using grep ... `pkg contents pkg:/SUNWgnome-display-mgr@0.5.11-0.111` did the trick:

Insert

DoubleLoginWarning=false

into the [daemon] section of /etc/X11/gdm/custom.conf

These are instructions for OpenSolaris, so on Linux YMMV, but it should be quite similar.

  1. That is, a kiosk configuration you can log into with a simple or blank password ↩