Double Jeopardy a requirement in the laws of many nations that a defendant cannot be put on trial twice for the same alleged crime based on the same evidence. Evidence is put before the court as to whether the defendant has been acqitted or convicted before of the offence under consideration. If either has happened the trial cannot procede further. In countries where there is no rule against double jeopardy oppressive and Totalitarian regimes can harass inocent people by putting them on trial repeatedly for alleged offences that led to acquittal before. For an example of harassment see Alexander Litvinenko.
Double jeopardy is what kept O.J. Simpson out of prison for over a decade, despite his book If I Did It. Fortunately, he wasn't too smart with his robbery and will be in prison for a long time.