Google hasn't said it they have done it(I don't follow Google that closely so I don't know if it is the Panda update or what). There have been several SO content clones. Some ranked higher than SO for given queries even after the update that was aimed at low quality content(the only reason I heard of it is because it generated a few HN articles). When that was mentioned on HN a couple of Googlers asked for examples so they could "Fix it".