Reference for Makkot 26:9

ור"ע מאי טעמא (דברים כה, ב) כדי רשעתו משום רשעה אחת אתה מחייבו ואי אתה מחייבו משום שתי רשעיות

And what is Rebbi Akiva's reason [for saying that kares liabilities are not subject to malkus]? [The Torah says] (Deuteronomy 25, 2) "according to his wickedness" which implies that according to one wickedness you can punish him but that you can not punish him according to two wickednesses.

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