Of course they can't and it isn't.They can be what they want to be. Its what you feel.
Is that an English rule?
I have friends:
Dad; English born White
Mum; Yemeni Israeli Dark skinned
Son 1; Born Germany Dark skinned Classes himself Australian. White Australian wife.New born son White English born now living in Australia.
Son 2; Born Germany White. Served Israeli Army He's and Aussie studying in Cyprus.
Son 3; Born Germany Dark skinned Served Israeli Army He's and Aussie, Living in Israel.
They can be what they want to be. Its what you feel.
If you have a British passport, you are also British.
He has not dual, but triple nationality, AMERICAN, AUSTRALIAN and DANISH.What nationality are you JD?