The 9,000s are listing of just the HD channels. They have also been remapped to the regular channel listing (for example, ESPN is 9424 and it is the same as the ESPN 140 HD station). They are just duplicates. Create a favorite list and use the list you prefer. So why have them? Well you can go to that station to make sure you are on the actual HD station. This is also where DISH first started to place their HD stations before they knew how to/had the technology to list multiple channels for the same station. They've kept it up since a lot of people like having all their HD in one area and not have to fiddle with adjusting their lists.
I'm not certain on the 800s but yes, they typically are duplicates. It is either because it is from a different satellite or because it is an MPEG2 station. Dish has switched all their HD over to MPEG4 though there may remain some MPEG2 equipment/subscribers out there.