Citizenship of the United States
A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.
United States passport
By law, a valid unexpired U.S. passport (or passport card) is conclusive (and not just "prima facie") proof of U.S. citizenship, and has the same force and effect as proof of United States citizenship as certificates of naturalization or of citizenship, if issued to a U.S. citizen for the full period allowed by law. U.S. law does not prohibit U.S. citizens from holding passports of other countries, though they are required to use their U.S. passport to enter and leave the U.S.