Hi @paulman,
Welcome to the community 
The drop down is connected to the Times collection and you have a relationship between the Times collection and Bookings collection to store the selected time for that booking!
I believe you need a condition that look like this!
All Bookings ( collection name ) Count > is equal to > 1 and a filter that Time ( relationship between Times collection and the Bookings collection ) > Time ( the property that the time stored in the Times collection ) > is equal to > Other Components > Time ( drop down name ) > Time.
And also adding this topic created by Yongki for prevent duplicate reservations! : Prevent Double Date Time Reservation
Thank you