Duplicate SPN’s – ADFS

Not any more. You sort of answered your own question. It has obviously been decided that there's no need for the original -A functionality of not checking for duplicates any more... but you can't just go willy-nilly ripping the -A out, because someone's script somewhere that was using -A would break.

Adding some official documentation to back up my claim:

http://technet.microsoft.com/en-us/library/hh831747.aspx

Changes to SetSPN

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In Windows Server 2012, SetSPN will no longer be able to register duplicate SPNs in a domain. When SetSPN –a is used, SetSPN will treat it as SetSPN –s.

For eveloping information about SetSPN, see Service Principal Names (SPNs) SetSPN Syntax (Setspn.exe) on the TechNet Wiki. For the command reference, see Setspn in the TechNet Library.

'https://adfs-dev.example.com/adfs/services/trust/2005/kerberosmixed' is not available as an endpoint on a proxy. Please pick an endpoint that is available.