Maintaining clean and accurate data is one of the most important things you can do to help your organization get the most out of Salesforce. Use Data.com Duplicate Management to control whether and when you allow users to create duplicate records inside Salesforce; customize the logic that’s used to identify duplicates; and create reports on the duplicates you do allow users to save.
Check the below link for setting up Duplicate Management in Salesforce
Duplicate Management Limitations
1. Duplicate management is available for accounts, contacts, leads, and custom objects. All other objects, including Opportunities and Person Accounts, are not currently supported.
2. Duplicate rules don’t run when records are created in following ways.
i) When records are created using Quick Create.
ii) When leads are converted to accounts or contacts and your organization doesn’t have the “Use Apex Lead Convert” permission.
iii) When a record is restored with the Undelete button.
iv) When records are added using Exchange Sync.
v) When records are manually merged.