Here’s some additional information that will help you understand how duplicate rules work and how to use them.
Available in: Salesforce Classic and Lightning Experience
Available in: Professional, Enterprise, Performance, Unlimited, and Developer Editions
Standard Duplicate Rules Starting in Spring ‘16, new Salesforce orgs come with standard account, contact, and lead duplicate rules already set up and activated. These rules define what happens when you try to save a duplicate record. Each standard duplicate rule has a corresponding standard matching rule that determines how two records are identified as duplicates.