I find myself sorely tempted to make a bad joke involving Alex Trebek’s long-lost twin. But I’ll spare you.

The protection against double jeopardy is contained in the 5th Amendment to the US Constitution:

[N]or shall any person be subject for the same offence to be twice put in jeopardy of life or limb.

But the US didn’t invent the concept; we just borrowed it from English common law. The general idea is that it’s unjust to try a person repeatedly for the same crime.

There are two major repercussions to this right:

1. Once someone is acquitted (found not guilty) of a crime, he can never be retried for the same offense. Period. Not even if three seconds after the acquittal is entered, he laughs evilly and admits his guilt. Not if new evidence is discovered. Not if God Himself comes down to Earth and says the guy did it.
2. Once someone has been convicted and has completed his punishment, he can’t be given additional punishment for the same offense.

So if you’re a prosecutor, you want to make damned sure your case is solid before you step into court, because if you lose, you won’t get a second chance.

But the right also has limitations.

1. Under something called the dual sovereignty doctrine, a person can be prosecuted by multiple jurisdictions for the same offense. Supposed George kidnaps Avery in California and drags him through Oregon and Washington before getting nabbed just south of the Canadian border. All three states can independently prosecute him for kidnapping, as can the federal government since the crime crossed state lines. He could get four separate convictions, four separate sentences. As a matter of policy, states will often decline to prosecute if the defendant is already facing hard time somewhere else—why spend the time and resources?—but that’s up to them.
2. Double jeopardy doesn’t stop a person from being tried if he commits the same offense again. Suppose George is acquitted of robbery and the next day he commits a new robbery. He can be tried for the new one.
3. Double jeopardy doesn’t stop a person from being tried for separate offenses. Suppose George not only kidnaps Avery, but also kills him. Even if George is acquitted of murder, he might still be tried for kidnapping. Law in this area gets sticky, so be careful. Suppose George stabs Avery and Avery survives. Now suppose George is acquitted of attempted murder—but two days later, Avery dies. Can George now be tried for murder? Probably, but it depends on the jurisdiction. Or suppose George kidnaps Avery, Barbara, and Carlos, all at the same time. If he’s acquitted of Avery’s kidnapping, can he now be tried for Barbara’s? And then Carlos’s? Maybe. Depends which rule the jurisdiction uses.

There are other complications too. Double jeopardy may or may not apply after a mistrial, depending on how and why the mistrial occurred. Double jeopardy doesn’t apply to civil matters (so even if someone’s acquitted, he can be sued) or to habeas corpus and other proceedings.

The takeaway for you as an author? Double jeopardy can make an interesting plot point, but take care how you use it.