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.
There are a lot of dual citizens, it’s nothing new or unusual.