John records Jesus saying “Amen, Amen” some twenty-five times. It’s typically understood as an oath formula, a “double witness” that stresses the truth of what Jesus says.
But there’s an additional dimension. The double Amen appears in the Old Testament only a few times. The people respond to Ezra’s blessing of Yahweh with “Amen, Amen” (Nehemiah 8:6), and several of the books of the Psalter end with a form of the double Amen: “Amen and Amen” (41:13; 72;19; 89:52)_
The most significant use, though, occurs in Numbers 5:22, n the instructions for the jealousy test. The woman who is accused of infidelity responds to the priest’s oath/curse with an “Amen, Amen.” By that response, she places herself under the curse of the rite, so that if she has been unfaithful she will become a curse among the people and her thigh will waste and her abdomen will swell (in a false pregnancy).
That is probably in the background in Nehemiah 8:6. Ezra reads the law, and the people, the bride of Yahweh, accept the full weight of Torah’s curses.
Jesus uses the phrase in a different context, it seems. He is not an accused bride, but an accusing husband, bringing the covenant lawsuit against Israel. Every time He says “Amen, Amen,” His words present another jealousy test for Israel.