There are instances when allowing double booking makes sense. For example, if you have multiple of the same Resource and do not want to create copies or if a resource can be used my more than one person at a time.
By default, Booked does not allow double booking, but this can be changed per resource. To allow multiple reservations at the same time for a Resource, open the resource management page from the Application Management menu. In the Access section of the Resource, there is an option to allow concurrent reservations. Simply set the total number of concurrent reservations for the resource and Booked will handle enforcing the limit.