Notify if Student Registers for a Course Already Completed Successfully

I'd like an autonotification if a student registers for a course that he has already successfully completed. (I'd rather this than prevent re-enrollments altogether.) Thanks, Mark * If this request is too similar to https://support.populiweb.com/hc/en-us/community/posts/214241447-Need-warning-notice-when-enrolling-a-student-in-a-course-they-have-already-taken-?input_string=Notify%20if%20Student%20Registers%20for%20a%20Course%20Already%20Completed%20Successfully, please merge ??