US – Philippines Dual Citizenship from an American Viewpoint
The automatic acquisition or retention of a foreign nationality does not affect U.S. citizenship; however, under limited circumstances, the acquisition of a foreign nationality upon one’s own application or the application of a duly authorized agent may cause loss of U.S. citizenship under Section 349 (a)(1) of the Immigration and Nationality Act [8 U.S.C. 1481 (a)(1)].
In order for loss of nationality to occur under Section 349 (a)(1), it must be established that the naturalization was obtained voluntarily by a person eighteen years of age or older with the intention of relinquishing U.S. citizenship.
US – Philippines Dual Citizenship from a Philippine Viewpoint
All natural-born citizens of the Philippines who have acquired any other citizenship are eligible to be a Philippine dual citizen.
A person is a natural-born Filipino citizen if s/he was born to a father and/or mother who was Filipino at the time of her/his birth.
Re-acquiring Filipino citizenship
Under Republic Act No. 9225, 2003, a natural-born Filipino who lost her/his Filipino citizenship through naturalization in another country may re-acquire Filipino citizenship by taking the Philippine Oath of Allegiance before a duly authorized Philippine official. The Philippine Oath of Allegiance does not require the renunciation of citizenship in any other country.
Prior to 2003 a Filipino could not have Dual citizenship
Residency in the Philippines is not required of those who wish to re-acquire or have re-acquired Filipino citizenship.