Depending on how your facility works you may or may not wish bookings to be able to checkin to a room on the same day when a booking is checking out.

KennelBooker can be set up to work either way, the default settings is to allow bookings to checkin and out on the same day from the same room/unit.

In the general boarding settings area of the Boarding settings page there is a setting called 'Only Book Runs when Completely Free'

To stop bookings from checkin on the same day a bookings is checking out simply enable this setting.