By default, DonorView looks for existing constituents in the database, matching by email address and name. If an exact match is found, no duplicate will be created. If you have existing constituents in the database that do not have an email address, a new constituent will be created since email address is the primary field that is matched. In addition, if an existing constituent match is found, and they have been unsubscribed, it will continue to keep the recipient unsubscribed.