New Active Directory Documents for IT Pros
Breaking guidance change: Although you can use Setspn -A, you should use Setspn -S instead because -S will verify that there are no duplicate SPNs. However, if you are using Windows Server 2003 or earlier, you will not be able to use the -S switch because it is not available for that platform. In the case where you cannot use -S, then you should manually verify that there are no duplicate SPNs by first running Setspn -L. Alternatively, you can load the software update for SetSPN on your Windows Server 2003 computer using Microsoft KB 970536(http://support.microsoft.com/kb/970536).
The Technet Wiki article Service Principal Names (SPNs) SetSPN Syntax (Setspn.exe) has been updated with this new guidance.