If the ics.conf parameter user.allow.doublebook is set to "yes", then:
Allowed — Double booking is allowed if the user’s doublebookingcalendar property is set to 1).
Disallowed — Double booking is disallowed if the calendar doublebooking property is set to 0. WCAP returns error 40 STORE_FAILED_DOUBLE_BOOKED.
However, if the user.allow.doublebook parameter is set to “no”, then double booking is disallowed, no matter what the calendar property is set to.