While it is illegal to have dual citizenship in Japan, it sounds like no one has ever been punished for it.
この場合、次のような言い方ができますよ。
ーWhile it is illegal to have dual citizenship in Japan, it sounds like no one has ever been punished for it.
「日本で二重国籍は違法だが、罰せられた人はいないそうだ」
to have dual citizenship で「2重国籍を持つ」
ご参考まで!