How could someone under age 18 have a dual citizenship if they were not born in the oher country?

If a Polish father -- who is a Jew -- and a British mother give birth in the United States, the child is a quadruple citizen right from the start, born with Polish, Israeli, UK, and US citizenship.

Since you are apparently really clueless, the way citizenship is obtained is visa jus sanguinis, loosely translated as blood relationship. Since a child has the genetic information (sperm cell + egg) of both parents, the child inherits the citizenship(s) of the father and the mother.

The "other" way to obtain citizenship is via jus soli, born on a country's soil. This is only accepted by a handful of countries, the US, Canada, and some Latin American countries.

Because his parents were Polish at the time of his birth he automatically acquired Polish citizenship when he was born. Since he was born in the US he would automatically have become an US citizen. He also might have become an US citizen automatically if his parents naturalized in the US before he became 18 in case he was not born in the US.

Since both countries allow dual citizenship his parents were still Polish even if they became naturalized citizens before he was born.

Actually, I do know a guy with dual citizenship with the US and Poland too...

Some countries will grant citizenship jus sanguinis (right of the blood) to people born of a citizen of that country, even if the child was not born in the country in which the parent holds or held citizenship.

Very few countries allow citizenship based on birth in the country, what's called jus solis, or citizenship based on "soil" or place of birth. US is one of the very few countries which allows this.

However, all persons have a natural birthright to citizenship in their parents' country of citizenship at the time of birth. If a baby's parents are citizens of Poland at the time of baby's birth, that baby is born a Polish citizenship.

ALL babies born in the US to non-US citizens have a birthright to citizenship in the parents' country of citizenship! By being born in US, they also have right to US citizenship. Therefore, all babies born in US to non-US citizens are actually dual-nationals.

It's entirely possible - indeed, it seems inevitably the case - that your friend was born in US to Polish-citizen parents. He is naturally a Polish citizen by birth to Polish citizens, but because he was born in US, also has a right to US citizenship.

Many countries allow you to get citizenship through your parents. A few even allow citizenship through grandparents. In fact that's how I got Irish citizenship. In the 1980s Ireland had overemployment (not enough skilled labor). To solve the problem they extended citizenship to third generation Irish descendants. My grandma on my dad's side was born in Ireland, so I applied and got it. Never set foot in Ireland in my life.

His parents registered him as an overseas Polish birth to ensure he got Polish Citizenship, being born in the US would give him birth right citizenship..............

Probably by descent.