What if a family I've moved to the Lead File ends up re-registering using the web registration form?

When a family is moved to the Lead File, they are no longer part of your main Jackrabbit database.  If they use the web registration form to register/enroll, a new family is created for them in your main Jackrabbit database.  This new family record has no bearing on the Lead File record for the same family.  You can leave the 'old' family record in the Lead File or choose to delete it.