Article 2, Section 1, Clause 5 of the Constitution of the United States:
“No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States.”
A person can meet the requirements of Article 2, Section 1, Clause 5 regarding being a natural born citizen while also being a citizen of another country. The two are not mutually exclusive.
My son, for example, is a natural born citizen of the US and a citizen of at least two other countries. He will be Constitutionally qualified to be President at age 35.