Whenever a sibling of a current student application, or if a parent applies for multiple children, we end up with duplicate or triplicate records for grandparents, siblings, and sometimes parents. We spend a huge amount of time de-duping, because when we ask our parents to fill in relationships, there is no way to link existing users.
I think this is a newer issue- usually, just like in the Inquiries, it'll recognize the user as having a preexisting relationship with the applicant and as being the same user as one that is already in the system. However, lately I've noticed that it'll add the user and create a new relationship, especially for parents. For instance, if a parent is listed in an application, the husband or wife that is in the preexisting account is listed as "spouse/partner" and the "new" relationship is listed just as "spouse." So the same account and the same user is tied to the "spouse" account in two different ways, creating a duplicate.