Kiddushin, Chapter Three, Mishnah Seven
A father has a right to betroth his daughter before she reaches majority age, defined by halakhah as being 12 1/2. Our mishnah deals with a father who says that he has betrothed his daughter but does not remember to whom.
1) [If a father declares,] I have given my daughter in betrothal, but do not know to whom I have betrothed her, and then a man comes and states, I betrothed her, he is believed.
2) If one says, I betrothed her, and another [man] says, I betrothed her, both must give a get; but if they want, one may give a get and the other marry her.
Section one: The father does not remember to whom he betrothed his daughter. At this point she is in a terrible situation; she could be married to anybody therefore she is forbidden to everybody, lest she be married to someone else. Comes along a man and says that he was the one who betrothed her. He is believed, since there is no contradictory testimony.
Section two: In this case two men come along and claim that they are the ones who betrothed her. Neither can marry her lest she is betrothed to the other. They must both give her a get. However, if they come to an agreement, one may give a get and then the other may marry her.