Short answers, pulled from the story.
Multiple citizenship is the legal status of being recognized simultaneously as a national by more than one country. It arises because no international convention governs nationality; each country sets its own rules under national law, and those rules frequently conflict with one another, allowing a person to satisfy the citizenship requirements of more than one country at the same time.
Countries that require automatic loss of citizenship if another is acquired voluntarily include Austria, Azerbaijan, Bahrain, China, India, Indonesia, Japan, Kazakhstan, Malaysia, Nepal, and Singapore, among others. Saudi Arabia may also withdraw citizenship if a citizen acquires a foreign nationality without the permission of the Prime Minister.
The Bancroft Treaties were a series of agreements the US negotiated with various European states from 1868 to 1937, under which the signatories pledged to treat voluntary naturalization as a renunciation of the person's original citizenship. They were named after American diplomat George Bancroft, who negotiated the first of them.
The United States taxes the worldwide income of its citizens regardless of where they live, one of only a very small number of countries to do so. As of 2023, US law exempts foreign earned income up to $120,000, and provides credits for foreign taxes paid to reduce double taxation.
Dormant or hidden citizenship occurs when a person holds citizenship in a country without being aware of it, typically through descent. For example, under Italian nationality law, a person born in Canada may hold Italian citizenship passed through an unbroken line of ancestors going back to a recognized Italian citizen, even if four generations removed, and may not know about it until they research their family history.
Effective nationality is the concept, recognized in international law, that a dual national has one dominant nationality for the purposes of international disputes, determined by factors such as habitual residence, family ties, financial connections, cultural integration, and armed forces service. The International Court of Justice defines it as a legal bond resting on a genuine connection of existence, interests, and sentiments. The Nottebohm case provided an important development in how tribunals apply the principle.