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.
Executive Order 13769
The United Kingdom's Foreign and Commonwealth Office issued a press release that the restrictions apply to those traveling from the listed countries not those that merely have their citizenship. The confusion led companies and institutions to take a more cautious approach; for example, Google told its dual-national employees to stay in the United States until more clarity could be provided. On January 31 the State Department updated the restrictions to allow persons holding dual citizenship to enter the US provided they possessed a US visa and entered using a passport from an unrestricted country.