[midoridensha]

People who were born with dual citizenship because of their parentage are a special case. They're supposed to make a choice by age 22 IIRC, but in reality you're allowed to defer it for a while because you're "considering it". So people in this situation just keep doing this indefinitely. Not really legal, but it's not really enforced much AFAICT.

For someone who gained JP citizenship through naturalization, it's a different matter. From what I've read, they will check to make sure you renounced your old citizenship, and will revoke your new JP citizenship if you don't (which probably also means no visa eligibility).