Can't pass the captcha challenge even if anwser is right

my problem was:
- go to captcha-protected post comment form (comment preview required / used riddler for captcha)
- fill in the correct captcha and all required fields
- submit (got the preview form)
- submit
- got message: The answer you entered to the captcha challenge is incorrect.
- change nothing and submit
- comment was submitted

I review the code, then fix some bug, especially the resubmitting bug.