“Account Already Exists” error on Windows 2000, XP, and Windows 7/Vista indicates that Active Directory already has the computer listed. You can not have two computers of the same name, even if one is offline. (If the other is online, you will see: “Duplicate Name on Network”. More about this further down)

How you fix it:
Log in to your Active Directory computer, and go to “Active Directory Users and Computers”. Find the computer name you wish to use and delete it.
Return to the computer, and set the computer name as you normally would. (Sometimes, Active Directory changes can take a short period of time before actually taking place. Also, if you have multiple Active Directory computers, you may run across the issue of having to wait for them to synchronize the change).

How to avoid it:
Before changing the name of any computer, you should drop the computer from the domain. The best way to do this is to switch the computer from being on the domain to being on a workgroup. The computer says: “I’m leaving” to the Active Directory server, and Active Directory drops the computer from it’s database.

Duplicate Name on Network:
You must find the other computer name and change it before you can use the name without receiving this error. You should still be able to log in and use the computer with this error, but it is very bad network practice to do so.