Microsoft Dynamics CRM's duplicate detection feature makes sure that your business data is accurate and clean by identifying potential duplicate records and letting you decide how you want to resolve these.
The softies recently published a new video that explains how duplicate detection works and shows how you can define criteria based on which Microsoft Dynamics CRM detects and notifies you of identical records.
Here are the videos:
Create and run duplicate detection rules (CRM Online users)
Create and run duplicate detection rules (CRM On-premises users)