Flag parent in registration process if they try to enroll same student for multiple classes at the same time

Our families have multiple children taking multiple classes. So the parent doesn't have to verify as much, I'd love it if, when they select the same student enrolled in conflicting classes (classes on the same day and time) there is a flag or pop-up that states, "This student is registered in XYZ on this same day and time. Which class would you like your student to attend on [DAY] and [time]?