Unique values based on more than one column that is not a text column

Hi, I have a sharepoint list that is used to keep track of people subscribing for multiple courses in a school. Sometimes people can't start right away, so we put them on a list and give them a call whenever there's a new space in a course. Sometimes they have to wait more than a year, and in the meantime they come back to subscribe again for the same course. Is there a way to prevent a user from being subscribed more than once for the same course in a sharepoint list. In that case, the combination of the student's name, surname and course (multiple choice list) needs to be unique. I find info on how to do this using powerapps, but only when the field format is 'text', not when one of them is a picklist. 

Anyone an idea?