Comment 45 for bug 329616

I have also found like K_Mux noted above that this happens to me when related to dual monitor support. I did no upgrades to xfce but rather switched my video driver. I switched from the open source radeonhd to open source radeon driver. The radeonhd driver always initialized my dual screen without me having to do a bit of work; didn't even have to put it into my xorg.conf file. However when I switched into the radeon driver it defaulted to mirror mode on my dual screen and multiple instances of the window manager appeared.

I can verify that clearing the .cache/sessions fixes the problem for that time only. (If I ever save the session and restart then the problem reoccurs.) When starting with a fresh session it still starts in mirror mode. I use xrandr --output DVI-1 --right-of DVI-0 to properly initialize my screen and then I am good to go until restarting X.

Another work around is for me to only have one monitor attached, startx, plug in the other monitor, enable and configure the other monitor. I have not tried configuring my xorg.conf to default to a configured state which may also solve the problem. But I can confirm that in my case this issue is dual monitor/xrandr setup related.