We look to Hollywood to give us fantasy, and the premise underlying the movie Double Jeopardy is just that -- a fantasy. In the movie (this is all in the trailer, so I'm not giving anything away), the heroine is convicted of killing her husband, who later turns out to be alive.

The movie tells us that she cannot be convicted of killing him again, so she is free to murder him in broad daylight. In reality, if our heroine kills the nasty husband now, she can be prosecuted, because the murder would take place in a different time and place than the (non-) murder for which she was earlier convicted.

The doctrine of double jeopardy does exist, and it basically says that you cannot be tried for the same crime twice. But if the two supposed murders didn't take place at the same time and place, they're not the same crime, simple as that.

As to your last question: The fact of the matter is that in rare cases, people can be convicted of murder without the body. The prosecutor can use circumstantial evidence -- for example, the fact that the victim has been long-missing and has never contacted her loved ones -- to prove that the victim is dead. If it weren't for this rule of evidence, potential murderers would have an all-too-easy way of avoiding prosecution -- by hiding the body. And next thing you know, Hollywood movies would be showing us creative methods of corpse disposal.