Not a joke at all, I have the second passport to prove it.
Your having a second passport, one issued by a country other than the US, is not dispositive of your assertion that "Any third generation person born in America to two parents who were born in America [and grandfather was a Euro citizen] is a dual citizen."
Many US citizens born in the US were and children of [both parents] naturalized US citizens. The heritage before the parents becomes irrelevant for citizenship purposes.
So, describe, with particularity, the circumstances that lead to your dual citizenship at birth. One of your parents was not a US citizen, I take it. But if both of your parents were born in the US, under Wong Kim Ark, they are at least citizens, even if not qualified to be president, regardless of the citizenship of their parents.