preexisting duty rule

a rule that prevents one from using a promise to do that which he or she is already duty-bound to perform by a separate agreement in order to create a new agreement. A little complicated at first glance, but this is what the rule says. If one party A is obligated to do task A under contract A, party A can't then create a new agreement, contract B, where party A gives his promise to do task A as consideration for the new bargain. Essentially the preexisting duty rule says that you can't spend the same dollar twice.