Commentary for Kiddushin 129:19

R. Nahman rules that two witnesses are necessary for kiddushin to be valid. But this seems to contradict our mishnah. If there were witnesses in the case in our mishnah, then his relatives should not be permitted to her—indeed, they are married. But if there are no witnesses, why would anyone think that her relatives are prohibited to him. According to the Talmud no one holds that kiddushin can be valid without any witnesses.
Rather, Rava posits that the case refers to one witness. This is a difficulty against R. Nahman who posited that with one witness no one thinks that the kiddushin have any validity.