I have dual citizenship (Japan and USA). I have been to Iran with my Japanese passport.
I would like to know if a US citizen with dual citizenship should hand in both passports when entering USA, and, if the other passport has arrival visa of Iran, can the US government deny me entry in the USA?
I would like to know if dual nationality US citizen should hand in both passports when entering USA
There's nothing requiring a US citizen to show foreign passports on entering the US, or even to mention other nationalities. As mentioned in other answers, there is a US law requiring you to "bear" a US passport when you enter the US, which strongly implies that you must also use it to do so.
As far as the immigration interview is concerned, once you establish that you are a US citizen, the interview is over and you must be admitted. (You can continue to be detained, however, for reasons related to the customs inspection, or for investigation of other crimes.)
I have never shown my foreign passport to a US immigration inspector, because I've heard a few anecdotes from people who've done so and had some degree of trouble as a result. I am always tempted to show it to see what kind of reaction I get, but so far my desire to get home and get some sleep has always prevailed over my curiosity.
From the CBP Inspector's Field Manual, which is available in at least two redacted versions online (emphasis added):
When you are convinced that an applicant for admission is a citizen of the United States, the examination is terminated. This is not to say that your role as an inspector is always completed at that time. Listing of the subject in a lookout system may dictate further action, such as notifying Customs or another agency of the person's entry.
It must be emphasized that the grounds of inadmissibility contained in 212(a) of the INA are applicable only to aliens. Consequently, the examination of a person claiming to be a United States citizen is limited to matters required to establish present citizenship. Once you are satisfied the person being examined is a U.S. citizen and any required lookout query has been completed, the examination is over.
Do not show your Japanese passport in the US. None of their business.
Do not show your US passport in Japan. You would lose you Japanese citizenship.
As others said, you cannot be denied entry in the US. Also, you have to enter the US with your US passport.
As a US citizen you cannot be refused entry to the US.
They may (theoretically) give you a living hell if they feel like it, suspect you of associating with terrorists, or whatever. But they can never refuse you entry into your own country.
As per the U.S Embassy & Consulates in Japan website:
Which Passport to Use
Section 215 of the Immigration and Naturalization Act ( 8 U.S.C. 1185) requires U.S. Citizens to use U.S. passports when entering or leaving the United States unless one of the exceptions listed in Section 53.2 of Title 22 of the Code of Federal Regulations applies. Dual nationals may be required by the other country of which they are citizens to enter and leave that country using its passport, but do not endanger their U.S. citizenship by complying with such a requirement.
