@elledd if you want credit from Duke for a summer class they come right out and say they don’t give credit for online courses. However if you want to use a summer online class towards your high school graduation requirements you need to speak to your own high school.
A general recommendation for DE classes is to save all syllabi. It is possible that the college you end up going to will ask for them when deciding to give any credit or not. That happens more when it is a class they aren’t familiar with but can happen for any class.