Sometimes, despite all best intentions, your outlook entries can become duplicated from merging PST files, misguided phone company personnel installing sync software, or a handful of other causes.

Exporting to CSV, removing duplicates in Excel, re-importing into a clean file will work, but can be an excruciating process.

ODIR (Outlook Duplicate Item Remover) is a great free tool that will remove duplicate contact entries from Outlook, as well as calendar and mail items.

Check out ODIR at http://www.vaita.com/ODIR.asp

This tool does not support 2013 or 64 bit versions of Outlook.