Validate field via AJAX call works only once ????

[Clamm]

Hi everybody,
i am trying to integrate recaptcha in our sign-up form. Everything
works at a glance. Well nearly everything.

The problem is that we use input field validation on every field in
the form. This works also for the captcha but only once. E.g.

1. Users enters the correct response <- AJAX calling recaptcha.lib
says "CORRECT"
2. Users leaves the field
3. Users focusses the field again <- AJAX calling recaptcha.libf says
"INCORRECT"

I've read nearly all i could find in the documentation but i assume
that each challenge only works
once. Can someone confirm that this is correct and that its not
possible to check a captcha twice?