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

These is a very common error in sharepoint 2003, so I've decided to post it here for future references.

Symptoms: When you try to add a user to a Microsoft Office Sharepoint Portal Server 2003 portal site or to a Microsoft Windows Sharepoint Services Web site that is hosted on the portal site, you receive the following error message:

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

Cause:
This issue may occur if the following conditions are true:
1. You create a user account in the Active Directory service.
2. You add the user to the portal site or to the windows sharepoint services web site.
3. You delete the user accoint from Active Directory. You then create a new user account that uses the same logon name as the user account that you deleted.
4. You try to add the new user account that you created in Active Directory.

Resolution:

To resolve this issue, connect to the Manage Site Collection Users page, and then remove the existing user account. After you do this, add the new user to the portal site or to the Windows SharePoint Services Web site. To do this, follow these steps:
1.
Use on of the following methods to connect to the Manage Site Collection Users page:
•
To connect to the Manage Site Collection Users page of the portal site, type the following address in the address bar of your Web browser, and then press ENTER:
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.
•
To connect to the Manage Site Collection Users page of a Windows SharePoint Services Web site that is hosted on the portal site, type the following address in the address bar of your Web browser, and then press ENTER:
http://ServerName/Sites/SiteName/_layouts/1033/Siteusrs.aspx
2.
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. Note If more than 100 users are in the site collection, the user may not be listed on the current page. To view additional users, scroll to the bottom of the list of users, and then click Next.
3.
Click OK when you are prompted to confirm the removal.

http://support.microsoft.com/?kbid=893696&FR=1