I'm Japanese, living in Yokohama city. 55 years old, sheet metal designer, sitting in front of a PC 19 hours a day using 3D CAd/CAM.
I didn't know double nationality is allowed.
As far as I know children who are born between parents with different nationalities have both their parents nationality up to when they are 20 years old. At the 20th birthday they have to choose one of their nationalities and leave the other.
This is because a child may be affected by different circumstances. The law waits until they are adult and can decide for themselves.
Maybe it's only Japan's law, and it's allowed in Europe, but I still can't understand how dual nationality is possible.
You should be lucky....