Let me try to explain.

/etc/xdg/xfce4/xinitrc installed by xfce-utils-4.7.0 runs `ck-launch-session xfce4-session`. so there's one.
then xfce4-session-4.7.0 runs a second instance. that's two.
i'm seeing:

Session4:
	unix-user = '1000'
	realname = '(null)'
	seat = 'Seat5'
	session-type = ''
	active = FALSE
	x11-display = ':0'
	x11-display-device = '/dev/tty7'
	display-device = '/dev/tty1'
	remote-host-name = ''
	is-local = FALSE
	on-since = '2010-09-07T15:39:06.957908Z'
	login-session-id = '4294967295'
Session5:
	unix-user = '1000'
	realname = '(null)'
	seat = 'Seat6'
	session-type = ''
	active = FALSE
	x11-display = ':0'
	x11-display-device = '/dev/tty7'
	display-device = '/dev/tty1'
	remote-host-name = ''
	is-local = FALSE
	on-since = '2010-09-07T15:39:06.981732Z'
	login-session-id = '4294967295'

if I remove the ck-launch-session from the xinitrc that xfce-utils-4.7.0 runs, I see:

Session3:
	unix-user = '1000'
	realname = '(null)'
	seat = 'Seat4'
	session-type = ''
	active = FALSE
	x11-display = ':0'
	x11-display-device = '/dev/tty7'
	display-device = '/dev/tty1'
	remote-host-name = ''
	is-local = FALSE
	on-since = '2010-09-07T15:34:01.150229Z'
	login-session-id = '4294967295'

notice that because of xfce4-session-4.7.0, it's all "active = FALSE" and therefore nothing works (like xfce4-power-manager, thunar's disk handling, etc.)