This article explores the Contact Duplicate Rules which can be configured and activated in your Salesforce environment.

Typically Contact Duplicate Rules will identify Contacts that are being saved with the same First Name, Last Name and Email Address, but this can be extended to include other duplicate detection logic, like phone number, street address, organisation, etc.

When a Contact record is saved the duplicate rule will run and provide an alert if a possible duplicate is detected. You then have the option of ignoring the alert and saving the new record or accessing the existing record.