Autrefois acquit is a plea made by a defendant who is charged of a crime or misdemeanor. It is a peremptory plea or a plea made before the commencement of a trial. A defendant can plead that s/he was tried earlier for the same crime under same facts of the case. This plea from a defendant can estop the government from carrying on with a trial against the defendant on the grounds of double jeopardy rule, which arises from Fifth Amendment of the U.S. constitution. However, the defendant should have been tried and acquitted in the previous case, since s/he was found not guilty.
Pursuant to 42 USCS § 2000h-1, no one can be put twice in jeopardy for the same act or omission. This means that a person can use the defense of autrefois acquit when s/he was tried and acquitted for a crime under the same facts. This is a procedural defense and so if a defendant takes this defense, evidence should be placed before the court substantiating the defense. Normally, the court can decide the matter as a preliminary matter, and on substantiating the plea, the projected trial can be prevented from proceeding.