Sometimes something goes wrong when saving sessions in xfce. For example it gave me three instances of NetworkManager today... Here's how to fix it:

Log out. Log in in text mode (control-alt-f1). Edit the xfce session: vi .cache/sessions/xfce-session-myhostname\:0

Move to the first line describing something you don't want to be started, and type d8d (which deletes 8 lines). Repeat until you got rid of all unwanted junk (so in my case leaving only one of the groups of nm-applet lines, and everything not related to nm-applet), and exit vi with :wq.

Log out (control-d), go back to X (graphics mode) (alt-f7), log in, log out with session saving enabled (just to be sure), log in.

It should be fixed now.