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.
Relinquishment of United States nationality
In absolute terms, few people relinquish U.S. citizenship or citizenship of any other developed country; in almost all countries, the number of people who give up citizenship each year is small relative to the total number of citizens abroad, let alone the total number of citizens in that country. In a 2017 study of citizenship relinquishments in twenty-eight countries, mostly OECD members, the U.S. came in sixth place in relative terms (i.e. relinquishers as a proportion of citizens abroad), behind Singapore, South Korea, Taiwan, Estonia, and Japan; and second in absolute numbers behind South Korea. The study author noted that emotional factors and prohibitions on dual citizenship may affect the rates of citizenship relinquishment, and that military conscription may explain the high rates in the other top countries besides the United States. Peter Spiro notes that most countries with military conscription provide exemptions for non-resident citizens. Regardless of the reasons, even for South Korea, the top country, only about 0.7% of citizens abroad relinquish each year, while for the U.S. the rate is only about 0.1%, though in both cases unreliability of population figures for citizens abroad means the rates are open to question. One former Foreign Service Officer, noting that State Department estimates of the population of U.S. citizens abroad have grown from 3.2 million in 2004 to 9 million in 2017, cautions that these estimates were "generated to justify consular assets and budget" and so "may be self-serving".