They sure do repeat the setup over and over and over and over and over and over and....
I asked about that once over on the MB fan club forum, and apparently there's a reason the producers do that. IIRC, it has something to do with the fact that the show is often shown in classrooms and maybe they only have time to show one or two segments per class, so it helps to recap the basics of the myth at the top of each segment.
Well, that's a darn rational explanation, then.
Always makes me feel like I'm being treated like an idiot with no - SQUIRREL! - attention span, though.