xfce4-session reports "Another session manager is already running"
when trying to start an xfce4 desktop inside a VNC session and another xfce4 desktop is running on the console.

Very crude workaround: Start the VCN/xfce4 session in a text console before starting a xfce4 session on the main console.

I found this when googling around for a solution:
"commit b0eec65b45fe4417351ca86f85095232df6d2f95
Author: Benedikt Meurer <benny@xfce.org>
Date:   Thu Dec 11 17:09:25 2003 +0000

    Added check to see if another session is already running."

Steps to reproduce:
  1) log into an xfce4 on the main console
  2a) start a VNC desktop with 'startxfce4' in the ~/.vnc/xstartup
  3a) Log into VNC, admire the blank screen and notice the error message in the VNC log file.
or
  2b) start a VNC session with only an xterm in the xstartup file (no window manager, or anything else)
  3b) try to start a xfce4 desktop with startxfce4 or xfce4-session

Actual result:
 error message in the VNC log file or in the xterm:
 "Another session manager is already running"

Expected results:
 A running xfce desktop session

Platform:
 Debian Wheezy with xfce4 4.10 from the Debian 'experimental'

This bug has also been reported in XFCE4.6 (Bug 7502) and XFCE4.8 (Bug 9916)