May as well throw this in since it seems relevant to the discussion. Recently YouTube has seen fit to show me two ads for each ad space. Maybe this has to do with the fact that I hit skip a lot, especially if the ad is too long and/or its in a bad part of the video and/or I've already seen the ad enough times.
...Now that I've typed that out, YT's ad behavior seems even more asinine. Huh.