Third party tools exists, otherwise, it only happens if you try to create a
new duplicate. Outlook doesn't have a way to scan for dups.
http://www.slipstick.com/addins/contacts.htm#dupes

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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
Coauthor, OneNote 2003 for Windows (Visual QuickStart Guide)
Author, Google and Other Search Engines (Visual QuickStart Guide)
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> There should be some implementation that will allow me to combine
> duplicate
> contacts already in my contacts folder.