How hard is it to acquire dual citizenship? I love the USA, but some friends belong to two countries. How?

It depends on the country involved, the relationship of that country with the U.S.A., and (usually) the circumstances of your own birth.

General example: if you were born in the U.S., but to parents who are citizens of another country, and that country is a U.S. ally, the U.S. State Department may allow you to accept citizenship from the other country without renouncing your U.S. citizenship. You would then be a dual citizen.

Each country sets its own requirements for visas, immigration, citizenship. Children have birthright citizenship in their parents' country/ies of citizenship. So if a child is born to two parents who are citizens of two different countries, the child generally has birthright citizenship in each parent's country. If born in US to foreign citizen, child has US citizenship AND birthright citizenship in parent's country of citizenship - dual-national.

The most common way to have citizenship pin two countries is to have parents from two different countries.

You had to have been born elsewhere, in a country that recognizes dual citizenship, or else have a parent from a country that gives dual citizenship to the children. The U.S. does not grant dual citizenship.

"Dual citizenship" is a scam perpetrated upon the real citizens of both nations.