These conversations tend to go all over the place for some reason.

The rule for the US is that if you hold US citizenship, regardless of any other you hold, you must enter on your US passport. If you were born in the US, you'll be presumed to have citizenship unless you can show otherwise.

If you were born in the US but aren't sure of your citizenship status you should check with the US consulate before trying to go to the US. If you want to give up your US citizenship, you have to go to a consulate and do it explicitly in most cases. If you don't, you're still a citizen.