26 Nicomedes of Segermi said: "My judgment is that heretics coming to the Church should be baptized, because they can obtain no remission of sins among sinners outside."
27. The answer to which is: The judgment of the whole Catholic Church is that heretics, being already baptized with the baptism of Christ, although in heresy, should not be rebaptized on coming to the Church. For if there is no remission of sins among sinners, neither can sinners within the Church remit sins; and yet those who have been baptized by them are not rebaptized.
Source: On Baptism, Against the Donatists (New Advent)