No, Evercontact does not create duplicates--- in its first version, it used to create a new contact for every email address, but based on user feedback, we've improved this.
As of 2015, here's the process:
- Evercontact detects an email signature & compares this email address (both the email address mentioned within the signature + the email in the from:) to find that email address within the contacts in your address book.
- If no contact matches that email address, then Evercontact looks for a contact with the same name (first name & last name).
- If still there is no contact match, then we create a new contact.
There is of course a chance that contacts with the same name may be merged, but most times, their emails will match up before this will happen & we've received zero feedback about this negative possible outcome. In general, this process is similar to Gmail's "merge" function which you can read more about here: https://support.google.com/mail/answer/165334?hl=en