If a student already owns a course or is a student in another one of your memberships you can add them to a new course manually through your Student Management Dashboard'.
Use either the student search function or search manually through the membership or course that the student belongs to in order to locate them.
Click on the students name.
From there click on the button located at the top right of the page that says '+ Assign course':
Once you've clicked on this button a slider will appear. From there you will be able to choose the membership and which new course you want to add your student to: