What is double jeopardy, and why do some people get tried twice?

The Fifth Amendment provides that no person shall be twice put in jeopardy of life or limb for the same offense. In practice this prevents retrial after an acquittal, retrial after a conviction for the same offense, and multiple punishments for the same offense. Jeopardy attaches once a jury is sworn or, in a bench trial, when the first witness is sworn. Critically, a hung jury or a mistrial declared for manifest necessity does not bar retrial, and neither does a defendant's successful appeal of a conviction, which typically permits a new trial.

The exceptions explain most cases that look like violations. Under the dual sovereignty doctrine, state and federal governments are separate sovereigns, so the same conduct can be prosecuted by both without violating the clause, which the Supreme Court reaffirmed in Gamble v. United States in 2019. This is how federal civil rights charges can follow a state acquittal. Different states can likewise each prosecute for conduct occurring in their jurisdiction. And a single act can constitute several distinct offenses, each separately chargeable if each requires proof of an element the others do not.

Civil proceedings are outside the clause entirely, which is why a defendant acquitted criminally can lose a civil suit over the same events, as in the well-known O.J. Simpson case. The standards differ: criminal conviction requires proof beyond a reasonable doubt while civil liability requires only a preponderance of the evidence, so the two outcomes are not logically inconsistent. Note also that the popular idea of confessing after acquittal with impunity is only partly right, since a confession could support charges for perjury, or for related offenses not previously tried, or federal charges.