You are being far too nice to these people. If they sent out double payments there was a need to stop payment on only half of them...not all of them. Get a corporate officer on the phone...either the president or treasurer and demand payment, give them a week, and if payment is not forthcoming...serve them with a summons.

If they refuse to put a corporate officer on the phone...tell them they have their choice of putting someone of authority on the phone to explain to you when you will be paid, or in the alternative they can explain it to a judge. That approach usually produces results.