When you close a membership that is linked to another membership (for example, a wife closes her membership but is joint on her husband's membership), CU*BASE automatically creates a non-member record so that joint owner the link can remain in place.  But later on if the wife were to come back and open a new membership, if the user chooses not to copy the non-member record but rather re-enter all of the wife's info from scratch, the non-member record does remain in the file.

There are also situations where a SSN and a TIN have the exact same 9-digit number, representing completely different entities. This is permitted as long as the Master Type is different between the records (MI for individual SSN vs. MO for organization TIN).  Otherwise multiple individual memberships that have the same SSN are assumed to be the same person (such as for tax reporting).  

Because of how the system needs to allow for possible exceptions to the rule, it is not possible to completely block someone from creating a membership that happens to match an existing non-member record, without getting rid of that non-member record at the same time.  But where a match is found, CU*BASE does at least present the SSN/TIN Already On File screen so the user can make a decision.