A new mishnah - a case of a woman who has her get, but has not yet received her ketubah. She's entitled to it, and he can't just claim he already paid it, at least not without taking an oath. Plus, a pruzbol situation. Note that in the "time of danger," when there were decrees against mitzvot, the Jews would destroy the documents, and being able to procure them became obsolete, during that time. Also, one can bring witnesses to attest to the fact that the couple no longer lives together, etc., if she wants to collect her ketubah after divorce AND death. Plus, the need for a receipt. And another mishnah: a woman who has 2 bills of divorce and 2 ketubot - remarriage to the same marriage, and divorce again.