8 USC § 1481 states that you may lose your citizenship for any of these reasons:
- becoming a naturalized citizen of another country, or declaring allegiance to another country, after reaching age 18
- serving as an officer in a foreign country's military service, or serving in the armed forces of a country which is engaged in hostilities against the US
- working for a foreign government (e.g., in political office or as a civil servant)
- formally renouncing one's US citizenship before duly authorized US officials
- committing treason against, or attempting or conspiring to overthrow the government of, the US
It used to be that doing any of those things automatically lost you your citizenship, but the State Department changed its tune in 1990 so that you have to tell the State Department that you intended to lose your US citizenship. Actually, before that, in 1986, Congress changed its tune enough to allow the notion that you might not want to relinquish your citizenship, but it wasn't taken for granted by the State Department until 1990. This means that dual citizenship is now legal in the US, and I'm sure that's what Mr. Soul has done.