session: skip adding duplicate DRM device
Use sudo /usr/bin/udevadm trigger --action=add to reproduce the issue
The handle_add_drm_card function in monitor.c calls wlr_session_open_file, which eventually triggers a DBus request (TakeDevice) to systemd(logind-session-dbus.c). This request fails, and as a result, systemd drops the master permission for the card