Can not add the user because a user with that name already exists

Problem:
Can not add the user because a user with that name already exists

Solution:
http://ServerName/_layouts/1033/Siteusrs.aspx Note The Manage Site Collection Users page is not revealed in the SharePoint Portal Server 2003 user interface. You have to manually connect to the page.
Click to select the check box that is next to the name of the user who you want to remove, and then click Remove Selected Users.

http://support.microsoft.com/kb/893696