פירוש על קידושין 54:5

At the basis of the mishnah in Kiddushin is the idea of “the extension of the oath,” gilgul shevuah in Hebrew. The idea is that if you are liable for an oath for one thing but not for another, since you must take the oath on one item, I can also impose on you an oath for the other thing as well.
Ulla claims that this idea comes from the repetition of the word “amen” when the sotah woman, suspected of adultery, responds to the oath administered to her. The second “amen” is an oath that she did not commit adultery with any other man. The Talmud will explore this and eventually show that one of these oaths is what we would call an “extension of an oath.”