Read: http://blogs.iis.net/brian-murphy-booth/the-biggest-mistake-serviceprincipalname-s
Try to delete duplicate SPN using below commands and create the SPN using the command again. Make sure you donot copy past the command type it some time extra space also create the issue for such commands.
Change SPN from machine account to domain user account to machine
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Remove the old SPN
1. SETSPN D <service>/<netbios name> machinename.domain.com
2. SETSPN D <service>/<fqdn> machinename
Add the new SPN:
1. SetSPN A <service>/<netbios name> <your domain>\<domain user account>
2. SetSPN A <service>/<fqdn name> <your domain>\<domain user account>
Verifying SPN's with SETSPN
1. SETSPN -L <machinename> (SPN should NOT be listed)
2. SETSPN -L <your domain>\<domain user account> (SPN will be listed)