The double jeopardy rule arises from the Fifth Amendment to the United States Constitution, the relevant clause of which reads:
“nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb”
This clause is intended to limit abuse by the government in repeated prosecution for the same offense as a means of harassment or oppression. It is also in harmony with the common law concept of res judicata which prevents courts from relitigating issues which have already been the subject of a final judgment.
There are three essential protections included in the double jeopardy principle, which are:
• being retried for the same crime after an acquittal
• retrial after a conviction
• being punished multiple times for the same offense
This rule is occasionally referred to as a legal technicality because it allows defendants a defense that does not address whether the crime was actually committed. For example, were police to uncover new evidence conclusively proving the guilt of someone previously acquitted, there is little they can do because the defendant may not be tried again—at least not on the same or a substantially similar charge. Fong Foo v. United States, 369 U.S. 141 (1962).
Although the Fifth Amendment initially applied only to the federal government, the US Supreme Court has ruled that the double jeopardy clause applies to the states as well.
Jeopardy attaches in a jury trial once the jury and alternates are impaneled and sworn in. In a non-jury trial, jeopardy attaches once the first evidence is put on which occurs when the first witness is sworn
(Source: http://en.wikipedia.org/wiki/Double_jeopardy#United_States)
