If anyone is interested in the facts.
1) There is no per se prohibition against holding dual nationality.
2) If you were born 1985 or later and hold dual citizenship you are required (but it is often not enforced at overseas embassies and consulates) to sign a statement at the time you apply for or renew a Japanese passport saying that you will endeavor to renounce your foreign citizenship, but there is no requirement to actually renounce your foreign citizenship. If you were born before 1985, there is no requirement to sign a statement.
3) Many countries (the US included) will not allow or recognize renouncement of citizenship if the reason is to comply with foreign countries' requirement for a single nationality so even if you wished to do so and signed papers renouncing foreign citizenship, the foreign country would just ignore it.