S1·E36 Sister Sister

Glinda the Good's twin sister, Melinda the Mean, plans to displace Glinda as the most loved witch in Oz. Melinda thinks being good can't be hard. But when each of Melinda's good deeds shows her true colors, Dorothy and the gang figure out there is an imposter! They have to find Glinda and prove to the Munchkins that she is the true Good Witch.