Reference for Ketubot 181:10

אלא לישמעינן כתובה נעשית מותר לחברתה ואנא ידענא משום דאחת בחייו ואחת במותו יש להן כתובת בנין דכרין

But he could have taught us that one ketubah is regarded as the surplus over the other, and I would have known that this is because where one [wife died] during her husband’s lifetime and the other after his death [the sons of the former] are entitled to the “benin dikhrin” ketubah?

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