Is the duplicate a fax number? Outlook considers fax numbers to be valid electronic addresses, since there are many client- and server-based components that can use such addresses.

One method to hide fax numbers from the address book is to prefix the fax number with one or more letters (maybe B for business fax, H for home, O for other). If the fax number begins with a letter, Outlook won't show it in the address book. This definitely works in Outlook 2000 and 2002.

There are a couple of utilities that can help you hide fax numbers from the address book. See http://www.rsoutlook.com/us/rshifa.htm and http://www.sperrysoftware.com/Outlook_Solutions.htm (Hide Fax Numbers).

Another way to avoid avoid having fax numbers appear in the address book is simply not to enter data in the fax fields. Put fax numbers in some other (non-phone) fields.