I'd say it does happen, but also the scenario described by a few people here. does not necessarily mean that shill bidding it taking place.
If you are outbid at the end of an auction and then offered another one at the same price, the seller might have two but wants to keep the price up by making it seem like there is only one. Once they know how much you are willing to pay, they offer you the second one. Works very well with concert tickets on the last day. If there are too many for sale at the same time, the price doesn't go as high.