Obviously that's redundant, Washington is the city and DC is the state abbreviation. Google doesn't say "Los Angeles CA., CA" for example. I reported this to them awhile ago -- if you right click on the map you can report an error. Recently I got a response: "The Google Maps problem you reported has been reviewed. We can't confirm that a change is needed. If you still see a problem, please tell us more about the issue"
Of course I told them more, because it's crazy. I also reported it via Google Map Maker, where you can edit and add things to Google Maps. It's not a big problem in the grand scheme of thing, but it annoys me!