Using a form, is there a way to have a message come up if a person picks a date that is taken?

I'm trying to figure out if there is a formula that can be used with forms that would alert someone if they pick a date that is taken already. have a sheet with a list of class dates, and I'm using a form to collect information from instructors that will tell me when they would like to hold a class, but if they select a date that is taken, I would like to alert them that the date has other classes scheduled on that same date.

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