I have never once been put in a a raid at the same point (IE 2/3, or 3/4 in DS/Terrace) that I had been put in previously, and I've run LFR every week since it was released in DS, many weeks on multiple toons.
I feel like people saying that that happens are 1) either making shit up, or 2) conflating the 1 time its happened into anecdotal evidence of it "happens all the time."