Recently, 2 proposals were closed because their subject was allegedly "covered by Super User":
These are probably not the only examples, even if I can't find others for the moment. (I will edit the question if I do.)
I don't think we get this problem very often with other existing sites, so I have to ask: Isn't Super User too broad? I mean, isn't it too broad for the potential users to understand its exact perimeters? Do we need to do something in order to make it more understandable?
My question is not about these two proposals. What I wanted to highlight is that these proposals, which have nothing in common (actually their subjects are really distant), both see their subject covered by the same site.
I found another proposal (which I actually created), also closed because the subject is covered by Super User: http://area51.stackexchange.com/proposals/32957/computer-peripherals.