II. Baptism by heretics must not be invalidated by second baptism.
But if it is established that a man has been baptized by heretics, on him the mystery of regeneration must in no way be repeated, but only that conferred which was wanting before, so that he may obtain the power of the Holy Ghost by the laying on of the Bishop's hands. This decision, beloved brother, we wish to be brought to the knowledge of you all generally, to the end that God's mercy may not be refused to those who desire to be saved through undue timidity. Dated the 24th of Oct., in the consulship of Majorian Augustus (458).
Source: Letters (New Advent)