A person can be a citizen of more than one country. A citizen of two countries is called a "dual citizen."
For example, US citizens don’t automatically lose their citizenship by becoming a citizen of another country. If you are a US citizen and you become a citizen of another country, you will have dual citizenship.
When a citizen of another country becomes a US citizen, they are allowed under US law to remain a citizen of their home country. But the law of their home country might revoke their citizenship.
Updated: February 2018