Microsoft Dynamics CRM 4.0 now includes a Duplicate Detection feature to protect the quality of your data.  Duplicate Detection rules can be defined for different record types, including custom entities, and can be defined across record types as well.  For example, a rule may be defined that states a Lead record is a duplicate if a contact record with the same phone number already exists.  When a duplicate is detected, the system will alert the user.  Regardless of the point of entry for data, such as bulk imports or synching offline data, an alert will be sent if a duplicate record is detected.

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Additionally, a system-wide duplicate detection job can be run, providing a list of possible duplicates which can then be cleaned by deleting, deactivating or merging records.

Out of the box Microsoft provides three rules that specify email addresses must be unique for Leads, Accounts and Contacts.