Dual citizenship is holding citizenship from two or more countries at the same time. It is not uncommon for many americans holding British and US citizenship at the same time. Citizenship is passed on to children by birth (jus sanguinis / right of blood). Your citizenship by birth, marriage or naturalisation, may have a different effect on your dual citizenship status.
People who are dual citizens often travel with multiple passports. A passport is a proof of your citizenship. For example, you are required to show your US passport when entering US borders and similarly British passport when entering United Kingdom borders.
In countries where dual citizenship is not permitted, traveling with dual passports is illegal. According to citizenship laws in these countries acquiring foreign citizenship may lead to automatic loss of citizenship.
Here are quick summary of countries that do not permit dual citizenship.
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Exception: Pakistan allows dual citizenship only with 18 countries. These are United Kingdom, Egypt, France, Jordan, Italy, Syria, Belgium, Switzerland, Iceland, Netherland, Australia, United States of America, New Zealand, Sweden, Canada, Ireland, Finland, Bahrain
Please see here for full list of countries: https://corpocrat.com/2016/10/06/dualmultiple-citizenship-which-countries-permit-dual-citizenship/