Double jeopardy is a procedural defence that prevents an accused person from being tried again on the same (or similar) charges following a valid acquittal or conviction.
If this issue is raised, evidence will be placed before the court, which will normally rule as a preliminary matter whether the plea is substantiated; if it is, the projected trial will be prevented from proceeding. In some countries certain exemptions are permitted, such as in the United Kingdom, where in Scotland a new trial can be initiated if, for example, the acquitted has made a credible admission of guilt, and in England and Wales, where serious offences may be re-tried following an acquittal if new and compelling evidence is found and for the trial to be in the public's interest. In some countries, including Canada, Mexico and the United States, the guarantee against being "twice put in jeopardy" is a constitutional right. In other countries, the protection is afforded by statute.
In common law countries, a defendant may enter a peremptory plea of autrefois acquit (formerly acquitted) or autrefois convict (formerly convicted), with the same effect.
The doctrine appears to have originated in Roman law, in the principle non bis in idem ("an issue once decided must not be raised again").
The 72 signatories and 166 parties to the International Covenant on Civil and Political Rights recognise, under Article 14 (7): "No one shall be liable to be tried or punished again for an offence for which he has already been finally convicted or acquitted in accordance with the law and penal procedure of each country."