I think it's safe to say, folks, that if you're going to create a Primarch, they don't need to be absolutely different from the canon Primarch. They do, however, need to be noticably different; for instance, copying Rogal Dorn wholesale under a different name wouldn't be allowed, but a nominally-similar character who happened to alight after the Scattering in circumstances that resulted in a vastly different personality would be more than reasonable.

By the way, [@Lord Coake], excellent video right there. Thanks for linking it here.