[everybodyknows]

Double-ended deals are legal in some US states e.g. California, and they are indeed an opportunity for breach of fiduciary duty. The way it works is that a crooked agent would get the seller to sign a listing contract that stipulates 3% to whatever agent represents the buyer irrespective of who that agent was working for. Then present only double-ended offers, or lowball outside offers to the seller -- hiding any higher offers that came through buyer's agents. And naive sellers have been defrauded this way.

The seller's defense is simple: Refuse to sign any listing contract that stipulates more than a 1% commission to a buyer's agent that is in any financial way affiliated with the listing agent's office. 1% being roughly in line with the actual effort required from the listing agent to deal with buyer-side coordination as well.

Such a stipulation doesn't quite close the door on collusion through favor trading, among agents in the local old-boy-and-girl network. But it's enough to shield you the seller from the grosser frauds out there.