Duo citizenship, as in American and Kenyan (or Indonesian) would also disqualify a presidential candidate. His loyalties would be split.
Mr. Obama has admitted that he was a dual citizen of Kenya, but claims that his Kenyan citizenship expired.
You can find this admission on his campaign website; the link is
http://fightthesmears.com/articles/5/birthcertificate
The relevant text is at the bottom of the main column:
Since Sen. Obama has neither renounced his U.S. citizenship nor sworn an oath of allegiance to Kenya, his Kenyan citizenship automatically expired on Aug. 4,1982.