Pope Formosus
The Roman Synod of 769 permitted and even prescribed the repetition of orders conferred by the antipope Constantine ("Liber Pontificalis", ed. Duchesne, I, 408 sqq.). In the ninth century, during the struggle with the "chorepiscopi", the ordinations conferred by those prelates were often declared null. In 881-82 Pope John VIII prescribed the reordination of Bishop Joseph of Vercelli, who had been ordained by the Archbishop of Milan, then under the ban of excommunication. On several occasions the ordinations conferred by Pope Formosus were declared null and were repeated.