I have run into such a double listing once and the fellow pulled the GB auction and kept the GA going.That's poor form to do both.

I would suggest you check all three sites before proceeding and report the individual who acts in this fashion-listing on GA and one of the bidding sites as well.

Some folks will set up an anonymous e-mail buyer to bid a minimum on auctions with no reserve as well but that's an entirely different issue--

Bottom line-
Some people have no class or ethics at all--