A German user originally reported this (in German) here (https://www.simplemachines.org/community/index.php?topic=571116.msg4042201#msg4042201):
In 2.0.17 (tested right here in this forum) when you have to CAPTCHA-verify your posts (f.e. as a new user), and you forget to click/solve the CAPTCHA before clicking Post, SMF complains (as expected) that the CAPTCHA wasn't solved1.
When you click the prominent Back link (really the only option you have), then solve the CAPTCHA (optionally even edit and/or preview the post before you solve it), and click Post again, SMF responds with "You already submitted. You might have accidentally double clicked or tried to refresh the page.", which is incorrect, no post was ever created.2
The denying behavior might be expected, because I assume the CAPTCHA might be invalidated by going back in the browser history, but in any case, the resulting error message is pretty misleading.
To be able to post again, the user has to (ctrl-c his message if he knows how,) leave the "new thread"/"new reply" page, f.e. by clicking back into the topic list, then start anew by clicking New Topic or Reply. (The original reporter even logged out and back in because he didn't see another option.)
[1]: Although the text could be improved upon, because it complains about letters, but a CAPTCHA nowadays is either a simple click or a "Where's Waldo?"-type set of pictures for me.
[2]: If the description isn't super clear, I have a 1.9MB GIF that I'll link to in the internal repo iff this will be addressed there.
does this happen on an install that uses the default SMF CAPTCHA? this site no longer uses that, so the issue here may be different.
Just verified on my test install, it works as expected there: First, the error message doesn't appear on its own page, just as a warning box on top of the editor, and if you solve the CAPTCHA correctly afterwards, you can post.
So the problem is limited to this site?
Yes. It was reported for this site (should have made this clear) and I could only reproduce it here.
Just a followup note that this was reported to the site team.