If they are auctions then the seller is only hurting himself by swamping the category.  If they are BIN and the seller is taking advantage of the free listings then each ID can only list the item once (or risk having them removed under duplicate listings).

There are companies that outsource the listing to eBay sellers and effectively dropship.  Each ID is a different seller even though their listings are identical.  The seller gets a commission. The items are shipped from the same location so it might appear as though they are the same seller, but they are not, so you would want to be absolutely sure before reporting them as it is within eBay rules to do this.