My cookies are clean. This happens also when you dont logout in the meantime and try to post a reply to two different forum posts. The issue is just always there.
From the site admin perspective it looks like there was a different captcha system before. Then the google captcha got build into the website and now it tries to read out data first time from something that does not exist any more.
EDIT: I did not have to enter the captcha two times when creating a new post(this one here). But i always have to enter it two times when i reply to a thread.