I've never had to sign any such agreement, but in one hangar I worked, there were two agencies based and one put their rate up and so most of the guys moved to the higher paying agent until the other agent also put their rate up to the same.
I don't know all the legalities of such an agreement, but I certainly wouldn't sign one unless there was some sort of compensating clause or bonus, ie if you fulfill this 6 month contract for our agency, you will receive X amount of cash at the end. Agencies make enough money out of our skills and I would resent to have my freedom of choice to work in a location I choose restricted in this way.
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NO FAULT FOUND