Not a goof (spoiler)

The way I see it, the fact that double jeopardy doesn't work like
people say in this film is not a goof, except in that it may be a
mistake by a character.

Libby is told about it by one of her fellow convicts. She got it
wrong, perhaps deliberately (in order to give Libby hope), perhaps
because she never really was much if anything of a lawyer.

Apart from that well, we know Libby believes what she was told.
Maybe she never looked to confirm it (it became her crutch, the
last thing she'd want to do is test it). But her believing it
is hardly a goof, particularly as we see how she came to believe
it (and from a source of unknown reliability as stated above).

The only other time double jeopadry is mentioned is when Libby uses
it to threaten Nick in the final confrontation. Travis confirms it,
to Nick, but he's just saying it to reenforce the threat. Travis
knows it's not true, and probably by that point he's explained it to
Libby too. The pair of them are just using it to put pressure on.

Make sense?