It might help to change the error message received when someone tries to renew and the email address is already in use. The current prompt asks for another email address. Many of my members do just that resulting in duplicate memberships.
Why not change the error message to say, "If this is membership renewal, please sign in to renew via your profile" or something like that.
The issue of renewals is a large one as we are midway through our first cycle of WA renewals and have had numerous problems. It has to really hit people between the eyes for them to get it.
John