If a man marries a woman who is observing the ʿiddah of her marriage to another man, in the event that both or one of them knew that her ʿiddah was not yet over and they knew that marrying a woman who is observing ʿiddah is unlawful, the woman becomes unlawful for him forever even if they did not have sexual intercourse after getting married. If they were ignorant about what ʿiddah is or it being unlawful to marry a woman who is observing ʿiddah, then the marriage contract is invalid.
Furthermore, if they have had sexual intercourse, it is forever unlawful [for them to get married to each other]; otherwise, it is not unlawful and they can get married again once the ʿiddah is over. Ruling 2420.* If it is established for a man that a woman is married, and he knows it is unlawful to marry a married woman but marries her anyway, he must separate from her and not ever marry her again. If he was ignorant of either matter – i.e.
the subject (mawḍūʿ) [the woman being married] or the rule (ḥukm) [of it being unlawful to marry a married woman] – then the marriage contract is invalid but the woman does not become unlawful for him forever, provided they did not have sexual intercourse. If he did not know she was married and had sexual intercourse with her after getting married to her, then based on obligatory precaution, she becomes unlawful for him forever.