Simple, getBIZZI simply cannot double-book.
Duplicate bookings are most likely to occur when you manually book items yourself, without the system being aware of it. If you use other – unsynchronized online agendas – besides your getBIZZI work agenda – getBIZZI will not know that you have blocked time with other activities. In case of synchronization, getBIZZI knows – after checking in your other agenda – whether these timeslots can actually be offered as free for services.