It’s not for me to say why Fassbinder and crew dubbed Margit Carstensen’s voice over Irm Hermann’s character in Effi Briest, but there is a fair amount of that going in this production. 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. To have made as many films as he did in the time he did, Fassbinder must have relied on whom he could get and when, and it looks like quite a few of the leads weren't available when it come to dubbing this epic.
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.