I was born Filipino and acquired a foreign citizenship through naturalization. Am I not automatically a dual citizen?

No. By becoming naturalized as a foreigner, you are deemed to have lost your Philippine citizenship. Republic Act No. 9225 or the Citizenship Retention and Re-acquisition Act of 2003 grants natural-born Filipinos who have lost their Philippine citizenship through naturalization in a foreign country the opportunity to retain or re-acquire their Filipino citizenship.