I actually like the idea of duplicate communities, because well... Let's say you want to talk Sherlock Holmes.
Well, some people don't want to discuss BBC Sherlock. Others don't want to discuss whether or not he's queer. And some want to discuss neither.
So to accommodate everyone, you WILL need 3 communities anyway. They may as well be on separate instances, so that if one goes down or isn't very active you can at least discuss the stuff that's allowed there.