Double jeopardy was a procedural defence that forbids a defendant from being tried again on the same (or similar) charges following a legitimate acquittal or conviction. In common law countries, a defendant may enter a peremptory plea of autrefois acquit or autrefois convict (autrefois means "in the past" in French), meaning the defendant has been acquitted or convicted of the same offence and hence that they cannot be retried under the principle of double jeopardy.
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, 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.