When two bookings are being made for the same Bookable Resource, at the same time, if I click the "Book" button (time slots will be searched) then the same time slots will be shown for both bookings that are being made. Now if I click on the same time slot and book it for the 2 bookings, even though the resource only has a capacity of 1, both bookings will be executed and then we will have 2 bookings for the same resource at the same time, and imagine that this resource is like a medic appointment, then we would have 2 customers going to the same doctor at the same time...
What I suggest is for Dynamics (msdyn_CreateRequirementGroupBookings action) to give an error back when a booking already exists for the same resource, in the same time slot, and depending on the capacity.