Hello,
As of now either , you can try deleting the clients from the Groups and the clients may re-register themselves.
OR
As provided in the Document work on the Workaround by Manually removing old duplicate client entries in the SEPM.
I am sure that would help you.
Again, I am sure once the Symantec Endpoint Protection 12.1 RU1 MP1 is released, the issue would be taken care off.