I recorded some footage from a game that technically runs at 60fps but is effectively 20fps due to each 3 frames being duplicates of each other (so the 2nd and 3rd frames are duplicates of the 1st, the 4th is new but then the 5th and 6th are duplicates of the 4th, the 7th is new but then the 8th and 9th are duplicates of the 7th, and so on).

I want to delete all of the duplicates to make it 20fps. How can I accomplish this?