The law is somewhat vague. You have a "duty to make efforts" (努力義務) in rescinding your foreign citizenship after electing Japanese citizenship. The Ministry of Justice has never clarified its interpretation of 努力 (i.e. what counts as "making efforts"?) I am friends with a few Japan-US dual citizens who have done nothing after electing Japanese citizenship; they are essentially gambling on a gray area of law.
Note if you acquire Japanese citizenship afterwards (or never had a second citizenship in the first place), then 国籍法11条1項 automatically voids your Japanese citizenship at the time you obtain a foreign citizenship.
Note if you acquire Japanese citizenship afterwards (or never had a second citizenship in the first place), then 国籍法11条1項 automatically voids your Japanese citizenship at the time you obtain a foreign citizenship.