What is double jeopardy?

What it actually bars

• A second prosecution for the same offence after an acquittal.
• A second prosecution after a conviction.
• Multiple punishments for the same offence in the same proceeding.

The exceptions people are surprised by

Separate sovereigns. A state and the federal government are different sovereigns, so both may prosecute the same conduct — the rule confirmed in Gamble v. United States (2019). Two states may do the same where the conduct crossed state lines.

Mistrials. If a trial ends in a hung jury or a mistrial declared out of manifest necessity, retrial is allowed — jeopardy has not terminated.

Appeals by the defendant. Win an appeal and the conviction is vacated, not converted into an acquittal; the state can usually retry.

Different offences. Under the Blockburger test, two charges are different offences if each requires proof of a fact the other does not — so a single incident can support several convictions.

A civil case is not jeopardy. Being sued for the same conduct is not a second prosecution, which is why an acquitted defendant can still face a wrongful-death claim.

Where it does not reach

Retrial after a conviction is overturned for a defect in the process, prosecution for a related offence that was not yet complete at the first trial, and professional or licensing proceedings, which are civil in nature.