also, exit stamps? Japan is really backwards in this way.
In 2019 I spent three months in Japan consulting for a very large and very recognizable Japanese aerospace company that was partnered with my employer.
Let's not focus on the bureaucratic nightmare my folks went through to make that happen, and the dozens upon dozens of paper forms I had to fill out daily/weekly.
Let's focus on the boxes and boxes of floppy disks they had on their supply shelves. Floppy disks that were needed, in 2019, to perform critical business functions.
Japan is spectacularly backwards in many ways.
She became a U.S. citizen in 2004.
After traveling back and forth between her home countries with both nationalities for years, she was flagged as being a dual citizen in 2017 — first at a passport office in Tokyo, where her passport was confiscated and her application rejected, and then by airport immigration officials when they realized she was exiting Japan with an American passport that had no entrance stamp.
So it took them 13 years to figure this out, and if she were actively trying to conceal her US citizenship, could probably have held both indefinitely.
You can still get around this by going via a third country, but then they can ask for a return ticket from that country etc. The only foolproof way to evade the system and keep citizenship in a 2nd country is to acquire permanent residence in 3rd country!
(Funnily enough the last exit stamp I got was from Gare du Nord in early 2020, as PARAFE eGates wasn't extended to US passports back then)
1) As an adult, consciously acquire another citizenship.
2) If you acquired another citizenship involuntarily as a child (eg. being born), at 21 you are required to choose one nationality. If you do nothing, you will stay Japanese on paper.