A duel is when two people fight for their life. It has nothing to do with citizenship. The US does not recognize multiple citizenship (dual, triple, quadruple, quintriple, etc.), so there will be no help from the US government's side for you to obtain another country's citizenship. You would not lose your US citizenship if you obtain the citizenship of another country. There are millions of US citizens who hold one or more other citizenships.
Yes. You can have Duel citizenship and keep your U.S. Citizenship. The only way you can loose your U.S. Citizenship is by officially renouncing it by letter.
You can but why? Europeans are particularly liberal meaning if you have something they want, they try and take it.