The contact importer de-duplicates by email address and optionally phone number, and works like this:If the file has an email address and the contact being imported has an email address, the importer looks up contacts by email address. If it finds a match by email address, it updates an existing contact; if it doesn't find a match, it adds a new contact.If the "also de-duplicate by mobile phone number" field is selected, first it tries to finda a match by email address. If it does match by email, it will update the existing contact. If it doesn't find a match, it will look up contacts by mobile phone number and update a contact if there is a match, or add a new one if there is no match. In all scenarios, it finds more than one contact that match, the system will add a new contact as a safety measure to avoid overwriting only one of the existing contacts.