Help with validation code

I have an app in which a person (logged in user) can book themselves onto an event and am trying to find out how to prevent them from booking the same event twice - does anyone have any code which would do this??

The Objects are:




As I see it there are two ideal options, firstly to remove events they are already booked on from the dropdown or, when the event is selected and is a duplicate, give them a message and clear their selection. In neither case should it be possible to save the record.

Did anyone in the community, or from Knack get back to you with a solution to this? I can’t find a thread?