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New orgs come with standard account, contact, and lead duplicate rules, which define what happens when you try to save a duplicate record. Each standard duplicate rule has a corresponding matching rule that determines how two records are identified as duplicates.
No setup is required for new orgs, but you can always turn off these rules at any time or create custom rules.
If you try to add a duplicate record, the standard duplicate rules are set to show an alert. As part of Duplicate Management, you can easily create a custom report type to review duplicate records that were added.
For more information about Data.com Duplicate Management, including detailed information about duplicate and matching rules, see Managing Duplicate Records in Salesforce. You can also check out this video: Managing Duplicate Records in Salesforce with Duplicate Rules .