If you get this error on your xbmc on the command line do the following

sudo nano /etc/sudoers

You will then get up a screen that contains the lines below many times

#
# XBMC-specific configuration
#
Cmnd_Alias SHUTDOWN_CMDS = /sbin/shutdown, /sbin/reboot, /sbin/halt
Cmnd_Alias MOUNT_CMDS = /bin/mount, /bin/umount
xbmc ALL=(ALL) ALL
xbmc ALL=NOPASSWD: SHUTDOWN_CMDS, MOUNT_CMDS

Just remove the duplicated lines and save and you should be good to go