In the Mishnah on Amud Alef, it says "All other women either drink or do not get a Kesuvah." We knew this already, what is the Mishnah adding that we didn't know?
Tzvi Levi Sherman, Manalapan, United States
Good question! Tosfos DH u'She'ar asks your question and does not provide an answer.
Tosfos Chadashim answers that the Mishnah wants to add that even if a Mamzer marries a Mamzeres she must drink, and if she does not she loses her Kesuvah (see Sotah 26a). The Chidush of this case is that one would think that we should be happy if she says that she won't drink, and we should give her the Kesuvah, since that is better than having her drink and then being blessed to bring into the world children who are Mamzerim. The Mishnah teaches that even so, if she refuses to drink she loses her rights to the Kesuvah.
The commentary Ahavas Eisan on the Mishnayos answers that the Mishnah is means to teach the following Halachah (which is true according to some): A person marries a woman who can't have children, but at the time of his marriage he also has another wife with children. The Mishnah is teaching that if the latter die after the Kinuy, we still apply the Halachah that the woman must drink or lose her Kesuvah.
All the best,