Starting with Outlook 2000, if you create an entry in a Contacts folder that matches an existing entry, Outlook asks whether you want to merge the information, ignore the new contact or save it as a new contact anyway.
There are tools (listed below) which will merge the data on duplicate contacts to one contact. This is useful if you are combining contact lists from different computers and the data on the contacts are not identical.
In Outlook 97 and 98, there is no built-in way to merge such duplicate records. You must make field-by-field comparisons to decide what needs to be updated in each record. An alternative method would be to code VBScript on an Outlook form to look for items with duplicate names and ask the user a lot of questions about how to combine those duplicates and resolve conflicting information. Unless you have a huge number of duplicate contacts, a little copy and paste from one Contact item to another is probably your best choice.
1-click solution to remove duplicate mails, contacts or calendar items. Merges contacts automatically, has an undo function and a configurable advanced mode. Version 3.03 Now supports Outlook 2010 (both 32 and 64 bit).
Free (up to 100 records) or Pro version (unlimited records) to clean up duplicate contacts. Lets you make field-by-field choices of which data you want to retain and drag and drop the data. Merges categories and other keywords fields. Supports custom forms. Works with Outlook 2003 to 2010. Version 4.