The website handles the mailing list as a CiviCRM group "Newsletter" (group ID 42).
The actual mail is sent through ConstantContact, and all of the unsubscribe notices are received there. The current newsletter mail list can be exported as a CSV file, then imported into ConstantContact. Unsubscribe messages received in ConstantContact must be used to update the mail list in CiviCRM.
Anyone can sign up for the newsletter: there is a link in the footer. Any email submitted here is being submitted to CiviCRM.
Admin can add people to the Newsletter group by editing the person's profile.
Duplicates are handled here the same way was any record in CiviCRM: Civi searches for a record based on the dedup rules (email address and name must match). Matching records are updated, new records are created. Names that are not typed identically (eg hyphen present/missing) will look like new records.
CiviCRM Employer relationship
When you open the person's civicrm record, there is a field called "Employed by". Filling in this field will allow you to select an existing record and create a relationship to it.
From a user interface, this field was creating duplicate records, and not linking to existing organizations. So, it was disabled, and created as a custom field. Once the relationship is created, the custom field is no longer used.