I was watching the fight between Goku and Kid Buu in English, and in a moment Goku says Kid Buu is "twice as powerful as he was before". Is this something of the dub or is this a literal japanese translation? Is Kid Buu twice as strong as Super Buu?
"Darn you, where's your weak point? You're twice as strong as you were before. Your speed, your power"
I am assuming this is the line you are talking about. Technically this could be considered filler as Goku fights Kid Buu using only Super Saiyan 3 in the manga and never turns Super Saiyan 2.
Also, this line would be with reference to the fat Majin Buu who Goku fought and not Super Buu as there is sufficient evidence to establish the fact that Super Buu is stronger than Kid Buu.