Japan is one of those countries that doesn’t allow dual citizenship and they require proof of renunciation. Something to think about for those from other “strong” countries who want to go down that route.
https://www.gov.uk/government/publications/form-rs1-guidance...
Japan doesn't allow dual citizenship if you acquire the japanese one later in life.
Plenty of people in Japan, like my nephew, have dual citizenship by birth. I don't know if they can keep both when they're adult. But as kids 100% guaranteed Japan allows it.
And then a great many mixed couple do, on purpose, give literally two different identities to their kids. One by registering them, say, in Germany with the name written in roman characters (say "Akane") and the other in Japan with the name written using kanjis (or kanas for that is allowed to).
I'm not talking about dual citizenships / two passports: I'm talking about the same person having two entirely different identities because his parents registered him, on purpose, differently in two different countries.
So, yes, there are literally a few people counted twice on earth. And you probably don't want to do that with the US citizenship (for US taxes its citizens worldwide and it's a reporting clusterfuck). But if you've got, say, a japanese identity and whatever european one: you're totally fine.
There was a lawyer, originally from Japan, living in the US who did try to force clarity on this issue and sued the Japanese government. Unfortunately she lost so the status quo remains. https://www.asahi.com/ajw/articles/15077494
Note if you acquire Japanese citizenship afterwards (or never had a second citizenship in the first place), then 国籍法11条1項 automatically voids your Japanese citizenship at the time you obtain a foreign citizenship.
> they are essentially gambling on a gray area of law.
It's not so much a gray area if they didn't do anything at all. The translation in English isn't on a vague side of legalize. One needs to go and try to renounce citizenship and keep the receipts, so if the motion is denied or you get sucked into a bureaucratic hell, you have a proof that effort has been taken, but the bureacracy prevailed.
It's interesting, that the word "effort" doesn't have a qualifier here, like "reasonable". Hazard guess -- the general language of the law somehow implies it, that if renouncing citizenship costs 1mm USD, you don't have to try unless it's pocket money for you or at all. If renouncing takes flying half the world to a country where things are happening , something should say ah nah.
So, for the Japanese government to a person to hold no more than one citizenship is beyond the rights held by the Japan as a nation. It cannot demand e.g. the King of England to revoke citizenship of a British subject, just because that guy is currently in Japan and on its own family register. If the UK's HM Government recognizes that guy leaving and converting, and amends his status accordingly, voluntarily, then that is just friendly international coordination and cooperation, and the sovereign rights of the UK is not violated.
And therefore, Japanese laws merely state its official stance of not allowing dual citizenship, and require applicant to make efforts, but it will not require the results of the effort to be exactly as it desires.
In an ideal world, there can be something like a "UN global citizen register" for every person on Earth, and the current nationality and/or recognized available nationalities can be fields in the global koseki printouts that can be printed out at a 7-11 everywhere. That would be very friendly to Japanese mental model of citizenship, but we're still maybe a century from there.
That's because it doesn't. I checked today. Japan requires that you renounce your US citizenship after you obtain Japanese citizenship, not before.
Usually they say:
“Here’s your new passport. You have X months to provide proof you renounced all your prior citizenships, otherwise we will revoke your citizenship. If we find out you had a 2nd undisclosed citizenship, we will revoke you then too.”
Some places give you and option of citizenship that is contingent only on renouncing the previous one. Here you can actually become stateless if you get the option, renounce the old one and never use it.
Some places just give a passport and a strongly worded letter to do or else.
It's of course ridden with loopholes added there on purpose, because life is complicated and operation manual for clerks that deal with it is ... something. I'm for example supposed to renounce my blue passport since I got a nice bordeaux colored one, but that obligation is suspended due to unfortunate circumstances outside of my control.
Sometimes you don't have to renounce if you are married to a citizen, sometimes your children get a free pass, but only if insist on filling in the form 42-2002/13477 which they don't like you to know about. Sometimes the cost of renunciation procedure or length of it are enough to pinky swear instead and send the passport to the embassy with registered mail and itemized contents bill.
Some people have two passports and only renounce (or promise to not use) one of them, thinking they outsmarted the system only to get it renounced 10 years later once they pissed off somebody more then they expected (the drama you didn't even knew existed: https://www.jpost.com/israel-news/article-910451 )