I may have found the answer to this here:
http://www.outlook-tips.net/faq.htm - but can't can't try it out until I get home from the office...
"I recently upgraded from the Outlook 2002 to Outlook 2003 and now have 2 Personal Folders, both with the same information
If you right click on the PST and receive an error when you choose Close and can't remove the extras using File, Data File Management, then you have ghosts. The easiest way to remove ghosts is to create a new profile.
1. Close Outlook.
2. Open Control Panel and find the Mail applet.
3. Select the profile and press Remove
4. Click the Add button and make a new profile.
5. After clicking finsh, select the new profile from the Mail dialog and choose Properties, then View or change email accounts
6. Select New Outlook data file and find your existing PST, add it to the profile and set it as the default
**Note that while the dialog appears to only create a new pst, you can select an existing one to use
For more information, see How to create a new profile