What is required if a man marries two sisters in two contracts but does not know which contract was first?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If the order of the two contracts is unknown, the man must separate from both women. This is obligatory because one of them is invalidly married to him, and the means to validate the other marriage (i.e., knowing which one is the permissible second contract) is uncertain. Separation from both, or dissolution of both marriages, is necessary to establish certainty of separation from the one whose marriage is certainly invalid.
Supporting text
The man has the option to separate from one, and then renew the contract with the other, irrespective of drawing lots. If he has not consummated either marriage, he may immediately contract with the other after separating from the first. If he has consummated one, he must divorce the unconsummated one, let her waiting period pass, and then marry the consummated one to ensure validity, assuming the consummated one was the second marriage.