From the Super Mario Wiki
2-Fold Goombas are a type of Goomba that only appear in Paper Mario: Sticker Star. In the overworld, they resemble ordinary Goombas, but in battle they become their true form. These Goombas can use sticker power to transform into a folded piece of paper with two Goombas on it. The only method of attacking is crawling toward Mario and biting him.
A 2-Fold Goomba is the first enemy that Mario battles to save the Traveling Toad. Even though the first battle with the 2-Fold Goomba shows two Goombas transforming into one, only one Goomba is necessary to become a 2-Fold Goomba.
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