Am I a dual citizen? Can I go to Japan, where I was born?

Citizenship of the Philippines is acquired by jus sanguinis (“by blood”), so the fact that your mother is Filipino automatically gives you citizenship of the Philippines (given your date of birth — rules have changed over time).

Citizenship of Japan is also acquired by jus sanguinis, though with a few additional conditions for people born abroad, which is not your case. Since your father is Japanese, so are you (for now, see below). The fact that you previously had a Japanese passport seems to confirm this.

So, at this time, you are indeed apparently a dual citizen of Japan and the Philippines.

According to the consular affairs section of the Embassy of Japan in the Philippines:

You probably want to get in touch with them if you have any questions. You’ll need to make an appointment anyway given the current situation.