Building a custom property list array

I have a List of webinar registrants.  The webinars are in different professional education areas.  If I import a CSV file of registrants with the custom property field Course Taken and the input on file import in that field being [“course_a”], when I later import another CSV file with Course Taken as [“course-b”] and the registrant previously took course_a, will the import override where [“course-a”] exists or will the import append and extend the list array to read [“course_a”],[“course_b”] ?