VII. About those who in fear or by mistake were re-baptized.
But as to those about whom you thought, beloved, we ought likewise to be consulted who were either forced by fear or led by mistake to repeat their baptism, and now understand that they acted contrary to the ordinances of the Catholic Faith, such moderation must be observed towards them that they be received into full communion with us, but not without the healing of penitence and the imposition of the bishop's hands, the length of the penance (with due regard to moderation) being left to your judgment, as you shall perceive the minds of the penitents to be disposed: in which you must not forget to consider old age, illness, and other risks. For if a man be in so dangerous a case that his life is despaired of, while he is still under penance, he should receive the gracious aid of communion by the priest's tender care.
Source: Letters (New Advent)