In our company we currently have a basic Nintex workflow with an acceptation / denial service for reservation of our company cars. (Check image Nintex Workflow Car)
The customer fills out the Nintex Form and the workflow starts, is there a possible way too automatically check for Double bookings or prevent double bookings? Sadly we can't use JAVA scripts in the advancement settings of Nintex Forms by company restricticions.
I have tried the method you mentioned, by setting the "Start day" and "End day" to be unique. However we have three different rooms which all can booked at the same moment. So sadly this isn't a valid options, since the timeslot will be claimed for all 3 rooms.