Under: Applications Menu >> Settings >> Session and Startup >> Application Autostart the application GKrellM has been added to automatically start at each session. However, when re-starting the computer TWO instances of GKrellM occur because the Session Manager is remembering applications that were running at the time of shutdown and re-starting an instance of the application meanwhile the Application Autostart is invoking an instance of the application because it's listed as being required to.
I sometimes have the same problem with gajim. I created a ~/.config/autostart/gajim.desktop file and save the session when I logout. Sometimes, xfce launches 2 gajim instances.
This is probably a duplicate of #9147
Yep, looks like a dupe. *** This bug has been marked as a duplicate of bug 9147 ***