I'm not sure about the long sales cycle, but I think everyone has this issue. I see it often when a prospect is considering multiple systems. They confuse questions and repeat requests. It can be frustrating, but that whole - the customer is always right might really be the best approach. I always try to use one email thread, even when they start a new one. This at least gives them the opportunity to read what they've already done.

I think that if you have good notes then this shouldn't be an issue. Give them notice that you'll include a 5-15 minute refresher part of your next discussion to go over what's happened so far. You probably don't want to be too pushy here because they might choose to stay quiet rather than ask again to avoid some kind of misguided fear. It's a relatable issue though - that's for sure.