It’s not for me to say why Fassbinder and crew dubbed Margit Carstensen’s voice over Irm Hermann’s character, but there is a fair amount of that going in Effi Briest. There is no doubt, however, that when Hermann opens her mouth, it is Carstensen that speaks, and I would only speculate on availability as the reason.
Due to the illness of Wolfgang Schenck, and a love of this particular project, Fasbinder spent longer on Effi Briest than he did on nearly any other film, and so this was most likely, as I have said, down to availability.
Above, Irm Hermann; who speaks pure Marget Carstensen. If you know these actors then in a way it makes the experience more enjoyable, however it can alienate you from the drama. For obvious reasons dubbing is more of an art in Europe than it is anywhere else, and Effi Briest is crafted somewhat differently from other Fassbinder films, put together in the artificial manner which most films are, layer by layer and one piece at a time.