Purchase Additional Content Through Membership Plugin

I'm currently using Membership for an online course site: each course = a unique membership level. However, I'm running into a problem when a user wants to purchase an additional course: Membership tells them they already have an account, and they need to create another.

How can I allow my currently registered users to purchase additional courses (memberships)?

Thanks!

http://www.socialmediaonlineclasses.com
WP single site
BuddyPress