From the Super Mario Wiki
During his battle with Mario and company in the game Super Mario RPG: Legend of the Seven Stars, Yaridovich, by using Mirage attack!, creates a double of himself, also named Yaridovich. The copy is identical to the original in appearance, but is weaker, having lower stats and 500 HP and attacking once it has been made. The copy also possesses different attacks than the original. In addition, the Psychopath thoughts of the double and the original are different. After one of the copies receives at least 500 damage total, rather than be destroyed, the clone and Yaridovich merge again. The two may also merge after a few turns. Yaridovich clones are the only bosses besides Exor that Geno Whirl can do 9999 damage to.