What documentation is it that you think tells one or the other country of the second nationality? It is not recorded on a Koseki Tohon nor a Birth Certificate. Parents' nationalities may be but that has zero correlation to the child's nationality given one can choose either.
Unnecessary worrying.
I'll admit, it's not high up the scale of things to worry about as a parent, but it's there - It has a little peeping voice you can just about hear on the rare occasions when everything else is quiet.
World is changing (anyone with a British passport is keenly aware of this) and there will always be reasons for governments to swap and share information on their own citizens.
That there is the potential for a future government to strip away a nationality isn't something I can just fuggeddabout. It was something I thought could be pushed aside for the next 12/14 years, not for the next 10/12 years.
The little nagging concern is compounded by a father who will never get British (or Spanish) Citizenship.
I know that it's rather different when a Japanese adult CHOOSES to become the citizen of another country, rather than a child not eventually renouncing citizenship they already had.
As I said, two less years of not worrying at all.
Snake bites are much higher up that parental worry list.