Shulchan Arukh, Even HaEzer 49

<b>קידש אחת מחמש נשים וכל אחת אומרת אותי קידש. ובו ג' סעיפים:</b><br>המקדש אחת מה' נשים וכתב לה כתובה ואינו יודע איזו היא וכל אחת מהן אומרת אני הוא אסור בקרובות כולן ונותן גט לכל אחת ואחת ומניח כתובה ביניהם ומסתלק ואם קידש בביאה קנסו אותו חכמים שיתן כתובה לכל אחת ואחת:

If a man is mekadesh (that is, betroths) one of five women, and he writes and delivers to her a ketuba (that is, a marriage document guaranteeing a certain amount of money to the women should the husband die or divorce her), and he does not know which one [of the five women] he married, and each [woman] says, "I am the one," he is forbidden to marry the relatives of all of them [as if each one is the "true" wife] and he gives a get (that is, a bill of divorce) to each one of them and he gives the ketuba money (that is, money given to a divorcee by the husband) between all of them and leaves [i.e., he leaves it up to the women to decide who was the "true" wife and, therefore, who has rights to the ketuba money]. If he was mekadesh her through engaging in sexual relations, the Sages punished him and he must give the ketuba money to each of the women.

המקדש אשה וחזר בו מיד ואמר שהמעות יהיו מתנה אפי' תוך כדי דבור אין חזרתו כלום וצריכה גט:

Someone who marries a woman, and retracts immediately, and says that the money is a gift, even saying so in the same breath, his retraction is meaningless, and he thus requires a bill of divorce.

כל מי שאינו בקי בטיב גיטין וקידושין לא יהא לו עסק עמהם להורות בהם שבקל יכול לטעות ויתיר את הערוה וגורם להרבות ממזרים בישראל:

Anyone who is not an expert in the make-up of divorces and marriages should not deal with them to teach (about them, because it is easy to err and to permit an erva [that is, a forbidden woman] and cause an increase in mamzerim [that is, children born from illicit relationships] in Israel.