Double booking of provider and time

I have just set up the A+ on a clients site. During testing l created a test appointment via the frontend user booking system and then was able to book via the backend appointments page the same provider with the same service at the same date and time.

Have l missed a setting to disable double booking.

Can l create a secure booking page on the frontend for the client to fill in. As they aren't very confident with WP backend. Has anyone done this before.