Double Jeopardy

Here’s one for the legal eagles out there:
In the movie “Double Jeopardy” a woman is [wrongly] convicted of murdering her husband, who is actually alive living under a false identity. After serving time & getting paroled she is under the impression that she can now legally kill him because the Double Jeopardy clause in the U.S. Constitution forbids being retried twice for the same offense. I always thought the DJ clause was about if you were found NOT GUILTY of something, it prevented the guv-mint from getting a second crack at you. Was this movies facts on the up & up, and are there any cases like this on the books?

The movie is basically crap:

DBL Jeopardy–the movie
double jeopardy (movie) - legal expert’s opinion?