Prevent double booking of dates

Hello
AT noob here, so apologies if this is very basic. We’ve set up a simple base that books staff against sites and dates. I want to make sure that we don’t accidentally double book a staff member on two sites on the same day. I have a field that logs each date a staff member is booked to work - I’ve tried to use deduce to identify any instances where are staff member has two identical dates booked, but I have so far not managed to make it work. is this the right way to go about it or is there a better way?