In 2011, federal authorities charged Calvin Walker, a Texas electrician, with 37 counts of fraud. Eighteen months later, Walker accepted a plea deal in exchange for all charges being dropped. That should have been the end of his legal saga. Yet two years later, Walker was again indicted for exactly the same alleged fraud, only this time by state authorities. He challenged this second prosecution as a violation of the Fifth Amendment, which guarantees that no person shall “be twice put in jeopardy of life or limb” for the same offense. But under a strange exception to the Double Jeopardy Clause created by the Supreme Court 60 years ago, both the state and federal governments are allowed to prosecute someone for the same act. Cato has joined the Constitutional Accountability Center in filing a brief urging the Supreme Court to review of Walker’s case and overturn this misguided “dual sovereignty” exception. We make three principal arguments. First, none of the Framers would have contemplated such a large exception to Double Jeopardy protection. Even before the Founding, English jurist and legal theorist William Blackstone wrote that it was considered a “universal maxim of the common law of England, that no man is to be brought into jeopardy of his life, more than once, for the same offence.” And in congressional debates before the enactment of the Fifth Amendment, Rep. Roger Sherman observed that “the courts of justice would never think of trying and punishing twice for the same offence.” Second, the practical magnitude of the dual‐sovereignty exception is much greater today than it was 60 years ago. For most of our nation’s history, the federal government left most criminal matters to be handled by the states; there were relatively few offenses punishable by both authorities. But in recent decades, there has been “a stunning expansion of federal criminal jurisdiction into a field traditionally policed by state and local laws,” as Justice Clarence Thomas wrote in dissent in Evans v. United States (1992). Now that nearly every state crime has a federal analog, the dual‐sovereignty exception risks entirely swallowing the Double Jeopardy rule. Finally, the Supreme Court created the dual‐sovereignty exception a decade before it held that the Double Jeopardy Clause fully applies to the states. Now that we know that it does, there’s no reason why a state prosecution shouldn’t “count” when a defendant objects to having been prosecuted twice. As Justice Hugo Black once put it, also in dissent, “If double punishment is what is feared, it hurts no less for two ‘Sovereigns’ to inflict it than for one.” Bartkus v. Illinois (1959). The Court should take this common‐sense advice and put an end to the misguided dual‐sovereignty exception, at least as it works in practice in modern times.