How can I make my scheduling form only accept one entry for a certain date/time?

I've created a scheduling form in hopes that only one person can sign-up for one time slot at a time. However, when someone picks a certain date/time it stays open making it available for multiple people to pick the same date/time. How can I fix this?

I've posted two screenshots above to help show my problem.The first one shows that a certain date/time is still available but the second one shows that someone has already picked that date/time. However, it's still available for others to pick. I don't want multiple people picking the same date/time.

I have been testing a cloned version of your form and I was unable to replicate the issue, I just submitted selecting an option and it was properly disabled, example: 

May you please kindly perform a test on your form and see if the problem persists? 

I have cleared your forms cache, please kindly test and let us know how it goes. 

It looks like it's still an issue. I just had two people sign up for the same time slot. 

May you please kindly provide us the submissions IDs for those ones you just received? 

This guide will help you getting such info: https://www.jotform.com/help/549-Where-to-Find-the-Submission-ID