[Nyororin]

The US does indeed recognize dual citizenship.

Japan does not. As soon as someone is considered old enough to make an educated adult decision, they are required to give up Japanese citizenship in favor of another country`s, or give up all others in favor of Japanese. It isn`t considered dual or multiple citizenship for a child - they`re just not considered old enough to make that decision on their own. In those sort of matters, it is up to the individual. A child can`t yet make that choice, so they are sort of excused for the time being.