Quaker Park Tennis Club, we are experiencing the same issue.  We are seasonal and all our members terminate each year and re-apply the following year.  There is absolutely nothing that we can find in order to identify who has purchased a membership, the only way we can tell at the moment is when someone sends us funds for a membership.  But this is external and time consuming.

I like the idea of the warning, as frankly the only way we appear to make the system work with our rules is to assign a member 2 separate memberships at the same time.