Canon 19 of Nicaea states: Concerning the former Paulinists who seek refuge in the catholic church, it is determined that they must be rebaptised unconditionally. Those who in the past have been enrolled among the clergy, if they appear to be blameless and irreproachable, are to be rebaptised and ordained by the bishop of the catholic church. But if on inquiry they are shown to be unsuitable, it is right that they should be deposed. Similarly with regard to deaconesses and all in general whose names have been included in the roll, the same form shall be observed. We refer to deaconesses who have been granted this status, for they do not receive any imposition of hands, so that they are in all respects to be numbered among the laity. (taken from
papalencyclicals.net/Councils/ecum01.htm)
I believe the topic of whether heretics should be rebaptised or not was also on the agenda (i.e., Canon 19 is not some frivolous canon, but one made to settle a specific disagreement). I have a book lying somewhere around my apartment which I could probably refer to (as it goes quite in depth), if I can find it.