Of course double bookings are not allowed. You don't want to arrive at a hotel and discover 2 couples booked the same room, no romantic weekend for you and your partner. So when client 2 wants to book an already booked record (although it seems to be available according to the GUI) an error "Room/contractor already booked. Please book another one." should be shown.
You should also make sure that the time a record is locked is as short as possible. So instead of already locking a record when the "book dialog" is shown (where you can enter the customer id), you should start the booking process (lock/update/unlock) after the user confirmed the "book dialog".
If client 1 terminates and has booked record 1, record 1 will remain booked forever (because there is no unbooking functionality). But it's quite normal a booking is booked forever (I booked a holiday at the end of October, so I hope it's still booked at the end of this month when I'm actually on holiday
)
