If someone has already been mistakenly punished for a crime they have not committed, are they then allowed to go and commit that crime (without punishment)? For example, supposing person A is charged with the murder of B but wrongly so as B was still alive. Does A then have the right, once he's finished his sentence in jail, to go and kill B? He has already been punished for it so you can't punish him twice for the same act!!

No, of course not! ...or so I first thought; but then your argument moved me; but now I again think... No, of course not! A's first punishment was probably unjust--certainly unfortunate. But if A now kills B, then A should be punished anew on any of the four halfway-plausible and at-least-sometimes-applicable justifications for punishment that I can think of. Deterrence: If we don't punish A anew, then we'll experience a crime-wave of similarly pardoned ex-cons (assuming certain ugly things about human nature). Rehabilitation: Well, incarceration didn't exactly work any criminal tendencies out of A, did it? (And it doesn't generally seem to accomplish much rehabilitation, at least in the U.S. .) But if incarceration would promote rehabilitation, which A surely needs, then A should be incarcerated. Public safety: Things didn't go so well when A was let out, so we're all better off keeping him in the slammer. Retribution: Here's where your...