Truth be told, having two passports in Japan is not necessarily a problem. If one lived a quiet life, one could conceivably keep renewing a non-Japanese passport ad infinitum. The USG permits dual citizenship and doesn't go out of its way to tell other governments about the nationalities of their citizens. However, as you know, I don't live a quiet life.

"How to Lose Your American Passport", Debito.org (2003-01-10)