GDM should do better if a user tries to log in multiple times
Right now, if a user is already logged in on their laptop and they try to log in again, GDM just jumps them to their existing session.
This is fine and great, but not always feasible. If the user is logged in remotely (say via XDMCP or gnome-remote-desktop's system daemon mode), or is logged in on a different seat (say with a DisplayLink device), then there's no way to jump to it.
In those cases right now, the behavior is less than awesome. it either tanks and goes right back to the login screen or its stuck in a limbo land gray screen it can never get out of.
We should add protocol to GDM to tell the login-screen gnome-shell instance about the issue and pop up a dialog to ask the user if they want to proceed by killing the other session.