Can a US citizen hold dual nationality?

Yes! There is absolutely nothing in the US constitution that prohibits a person from holding both a US passport and that of another country. To be sure the IRS does not like it. Neither does some other government agencies, but it is not a violation of the law for someone to pursue second citizenship. You will not lose your US citizenship if you obtain another passport. In fact, for tax reasons, the US government would like to see you remain as a US citizen (regardless of any other passport you may have) so they have the opportunity to tax you to death regardless where in the world you are living.