[A Man Married a Woman, Then It Became Clear They Had Been Breastfed by the Same Woman]

Answer—and Allah grants success: In such a matter they should have referred to mujtahid scholars, for there is disagreement among scholars in some questions of breastfeeding; and when that is so, annulment requires the ruling of a judge.
However, since the annulment did in fact take place, and the subsequent marriage did take place, we say: the fatwa of that student to the couple—annulling the marriage from its root and declaring it void—takes the place of a judge’s ruling. That is because, as appears, this student was the religious reference for the people of that region; they trusted him; his word was accepted among them; and his fatwa was decisive.
Moreover, it was he who ordered separation; therefore we have treated him as standing in the place of a judge. This is our carrying the actions and transactions of Muslims upon validity. Accordingly, the woman’s marriage—after her womb was cleared by delivering—was valid, and the prior annulment was valid; based on that, there is no harm in what occurred.
Yes: in the marginalia of the madhhab it is stated that if the two spouses mutually agree to annulment, believing the marriage to be invalid, they may annul without needing a judge’s ruling.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1