The service principal name (SPN) that this instance wishes to register is not unique within the enterprise. In order for ...

The service principal name (SPN) that this instance wishes to register is not unique within the enterprise.  In order for mutual authentication to work with this instance, these duplicate SPNs must be removed.  Please note that these duplicate SPNs can be registered in any trusted forest on any account. 
 
SPN: 
%1 
 
User Action: 
Locate all accounts which have the identical SPN string on the servicePrincipalName attribute and remove it from those accounts.