The prohibition against being tried twice comes from the U.S. Constitution, Fifth Amendment, which provides in relevant part: “…nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb…”
This protection is now understood to protect against both a subsequent trial for the same offense following an acquittal, and multiple punishments for the same offense following a conviction.
It does not matter if new evidence comes to light – once an acquittal is reached, a videotape might emerge showing the accused committing the crime, and he can’t be retried.
However, the issue of what constitutes “the same offense.”
For example, if I waltz into a Falls Church convenience store with a gun, fire it into the ceiling to gain the attention of the proprieter, shove him behind the counter, demand the money from the register, take it, and leave, I’ve clearly committed the crime of armed robbery. But I’ve also brandished a firearm, discharged a firearm in the city limits, committed simple theft, and assaulted the shopkeeper.
If I’m tried and acquitted for the armed robbery crime, may I be tried again for the crime of assault? Of theft? Of the brandishing a firearm charge?
The answer is… it depends.
The seminal case for this determination is Blockburger v. U.S.. This case created the rule that where the same act or transaction constitutes a violation of more than one statutory provision, the test to be applied to determine whether there are two offenses or only one is whether each provision requires proof of an additional fact which the others do not.
Theft, then, is a “lesser-included” offense of armed robbery under these facts. In order to prove theft, the state would have to show that I stole something; in order to prove armed robbery, the state would have to show that I stole something while armed with a weapon. An acquittal of the armed robbery charge means an automatic acquittal of all lesser-included offenses; I cannot be retried for simple theft after such an acquittal.
On the other hand, discharging a firearm within the city limits is not an element of armed robbery. So theoretically, I could be retried for that crime, assuming I wasn’t charged with it at the first trial.
There might well be problems of proof at that second trial, though, as the state might be precluded from telling the jury the whole story of why I was in the store. That’s a discussion for another thread, but from a strict double-jeopardy standpoint, it’s possible.
Either side, by the way, may request that the jury be asked to consider a lesser-included offense, if the evidence supports it. So the prosecution might charge me with both theft and armed robbery - if the jury convicts on both, the two convictions merge into one. But they might choose to acquit on armed robbery and convict on theft. Sometimes juries do funny things – I’ve seen cases where they acquit on theft but convict on armed robbery, obviously not understanding their instructions. This usually means an appeal claim that the verdicts were legally or factually inconsistent.
Anyway, enough rambling.