https://docs.mongodb.com/manual/tutorial/control-access-to-mongodb-windows-with-kerberos-authentication/

Change these two lines:
setspn.exe -A <service>/<fully qualified domain name> <service account name>
setspn.exe -A mongodb/testserver.mongodb.com mongodtest
To:
setspn.exe -S <service>/<fully qualified domain name> <service account name>
setspn.exe -S mongodb/testserver.mongodb.com mongodtest

per setspn.exe docs
"Replaced references to using the –A parameter with reference to use –S instead. The –S parameter verifies that no duplicate SPNs exist before adding a new SPN."

This is also to have consistency with our docs on page https://docs.mongodb.com/manual/tutorial/kerberos-auth-activedirectory-authz/ which shows the command as "-S"