Originally Posted by
joejones
This is correct, but if you were born with both Japanese and foreign citizenship, you never naturalize in the foreign country so it isn't an issue.
Law says you may not hold two. So yeah, it kind of is an issue. If you decide to worry about it. Most don't.
As for how they would force you to give up your other nationality you should carefully read Articles 14 and 15 of the Nationality law. It says zilch about douryouku.
The two conditions, i.e. declaring a foreign nationality and the law saying you cannot have one are at odds. One or the other must be right. They cannot both be.