Did they renounce their US Citizenship? They should be required to do so. Also, they should have to get in the queue for green cards to return.
I’ve been to 2 Naturalization Ceremonies. During the ceremony you renounce the citizenship and allegiance to all other countries. Then you take the oath of allegiance to the US.
I don’t know if it’s the same for other countries. The concept of a dual passport should not be allowed. I know several people that have two passports, mostly with Canada. I tell them that they violated their citizenship oath. They just don’t care.
Actually, they can’t. A natural born American holds citizenship by right. Unless he explicitly renounces it, it cannot be taken. The Congress can grant citizenship, but it cannot take it away.