Where, in an action to recover an alleged double payment of a real estate agent's commissions, defendants claimed that the second payment was for other services, and that the receipt executed therefor had been given to plaintiff's agent, who had knowledge of the previous payment of the commissions, an admission of such prior payment, and the execution of such receipt did not shift the burden of proof to the defendants to show that the second payment was not for services previously paid for. Phelps v. Miller (Tex. Civ. App. '04), 83 S. W. 218.