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(

The Technet Wiki article Service Principal Names (SPNs) SetSPN Syntax (Setspn.exe) has been updated with this new guidance.