I'm running OpenSUSE Leap 42.2 on my desktop and both my notebooks. After the last system update, one of the notebooks started to show the following Error dialog after logging into LXDE:
GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: An authentication agent already exists for the given subject
Googling for the message didn't help much, so I had to start experimenting myself. Comparing the output of ps -eo euser,pid,cmd | grep [p]olkit on different devices showed the notebook had one more line there:
polkitd 1657 /usr/lib/polkit-1/polkitd --no-debug
choroba 2283 lxpolkit
choroba 2308 /usr/lib/polkit-gnome-authentication-agent-1
The extra line was the last one.
I had no idea who started the gnome agent, and I wasn't able to find out quicky: pstree shows systemd as its parent.
Nevertheless, I was able to prevent it from running. In the file /etc/xdg/autostart/polkit-gnome-authentication-agent-1.desktop, I changed the line
The error is gone.