He could not be a dual citizen in USA and Indonesia. he lost his US citizenship when he became a citizen of Indonesia. At the time there was no dual citizenship with Indo and they may still be true today.
Some argued that a child cannot lose it but I think that is incorrect.
I don't doubt, however, that Barack Obama is presently a US citizen. He carries a US passport.
He may have re-acquired his citizenship. Perhaps he never lost it to begin with. I'm sure the March raid on Obama's passport file had something to do with this.
His current citizenship is not at issue. What's at issue is, if you are "born in the USA" can you EVER lose or surrender "natural-born" status?
I doubt it.
I think the passport issue is also relevant. At 20, if Obama traveled on an Indonesian passport (do we know this?) then it is more damaging to his claims of US citizenship