In the United States, the Fifth Amendment's Double Jeopardy Clause (incorporated against the States via the Fourteenth Amendment's Due Process Clause) has been interpreted to have the following two consequences (among others):
1) The prosecution cannot appeal an acquittal;
2) The prosecution of a defendant (whether it results in conviction or acquittal) by Jurisdiction A does not bar his subsequent trial for the same acts by Jurisdiction B, where A and B are "separate sovereigns." This rule was dramatically illustrated when the police officers who were acquitted on California state law charges of beating Rodney King were subsequently tried on federal charges. California and the U.S. government are separate sovereigns (as are different U.S. states, but not different local governments within a state).
Meanwhile, as dramatically illustrated by yesterday's rulings in connection with the Madrid bombings, Spain has the exact opposite set of rules: 1) Prosecutors can appeal an acquittal; but 2) the fact that a defendant (here Rabei Ousmane Sayed Ahmed, aka "Mohammed the Egyptian") has been convicted for the same conduct now charged by a different sovereign (here Italy) does bar his trial.
The juxtaposition of these rules does not, of course, mean that either system is necessarily superior. The prohibition on appealing an acquittal in the U.S. is no doubt strongly tied to our reliance on juries in criminal cases. It protects the jury's role as finder of fact and conscience of the community. Spain, using bench trials within an inquisitorial system (in a non-pejorative sense), attributes less significance to the conclusions of the trial adjudicator.
Likewise, the separate-sovereign rule is a tricky matter. There can be something very unfair about its application to permit essentially a second bite at the apple, especially when the first trial resulted in an acquittal. Yet doing away with the rule completely could prove problematic where the first prosecution is either incompetent or corrupt. Suppose that during the Civil Rights era, local prosecutors rushed to try the members of a racist lynch mob so as to get an acquittal, and thus foreclose the possibility of a trial on federal civil rights charges. The separate sovereign rule makes such a ruse unavailing.
More broadly, the juxtaposition of the U.S. and Spanish approaches to what we call double jeopardy shows the context-dependence of judgments about procedural fairness. There is a strong tendency to confuse what's familiar with what's best.
Posted by Mike Dorf