By default the system allows Technicians to be allocated to more that one job over the same time-slot. This helps in situations where:
you are running concurrent install jobs and have technicians working between them depending on the days workload or completion status
you need flexibility to call someone off a job to attend another; ie an urgent job; ie because they are on call or are a senior attending other technician jobs for sign-off
If you do not want this setting and to be presented a warning when you attempt to assign a Technician that is already booked (partially or fully) then watch this brief video that shows you how to disable it.