sneaky little clause that one.
well if you can get the buyer to come through the first agent then it would only cost that one commission, and you get to sell the house. or tell the agents to sort it out between themselves or you will cancel the contract and sell privately to the buyer, no commision.
also all contracts have to be registered, most agents dont do this, so if you really want a get out, then you could get your solicitor to look into it, if the contracts not filled out correctly with the house owners names and registered etc then maybe you can cancel with them easilly