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Ne bis in idem is a Latin phrase meaning “not twice for the same.” This phrase was first used at common law to describe a legal concept called “double jeopardy.” Double jeopardy does not allow a person to be charged with the same crime arising out of the same incident more than once. There are some exceptions to the concept of ne bis in idem, such as some circumstances in which there is a successful appeal by a convicted defendant.
Once a defendant has been acquitted — i.e., found to be not guilty — of a particular crime in a particular instance, he or she may not be retried for the same crime. Ne bis in idem is a concept in place to protect citizens who have been acquitted of a crime from having to deal with suspicion of that crime for the rest of his or her life. For example, if ne bis in idem were not applied to common law, then anyone suspected of a particular crime would not be able to escape the government’s pursuit until the governmental agency decided to move on. He or she could theoretically be subjected to perpetual prosecution until a jury found him or her guilty.
There are some exceptions to ne bis in idem, however. For instance, if a criminal defendant is found guilty, but the guilty verdict is turned over on appeal, in some cases he or she may be subject to another trial. Whether or not the defendant may be retried for the same case at that point usually depends on the reason for the overturning of the verdict. In many jurisdictions, if the reversal is based on the weight of the evidence — i.e., that the record as presented could not support a guilty verdict — then the case may be retried. Conversely in those same jurisdictions, if the reversal is based on the sufficiency of the evidence — i.e., that even if all the evidence presented in the case is looked at in a way favorable to the prosecution, it is not enough to prove every element of the crime — then double jeopardy remains intact and the defendant may not be retried.
Double jeopardy does not attach in the event of prosecution by separate sovereigns. For example, in the United States, state governments and the federal government are considered separate sovereigns. So, if someone is charged for a federal crime that also constitutes a state crime, then he or she may be subject to prosecution in both state and federal court for the same offense.