Prevent data entry if data exists?

[phn]

Hi,

I have created a booking database which allows the user to input bookings for loaning of camera systems.

There are 5 camera kits that can be loaned, what I need to do is prevent the user from double-booking a kit.

If a kit has already been booked out, I want an error message to appear when the user tries to add a booking for that kit, is there any way of doing this?

Any help would be appreciated.

Thanks!