Overlapping time slot

I have set up times slot so booker can pick the time they want to schedule. But when a booker selected a time slot that time slot remains available to other bookers to select. So the calendar overlap bookers. How can we make sure that when a the time slot was selected that it no longer available for other bookers to select.

You will want to ensure that you have units per slot set to 1. This will ensure that no one can book a time that is already booked by someone else. More info here..