Eliminating double bookings

I have a calendaring application that uses asp to connect to an access database. The application will allow multiple users at a single time. My question is this:

How do I eliminate double bookings from happening if 2 users select the same time on the same date for the same item? I run a query before I show the available times to the end user but if they select the same things at the same times and submit it at the same time it allows 2 records for the same timeslot to be added to the table.

Does anyone have any suggestions on how to handle this?

Thanks in advance,
Scott