First, I'm brazilian, and so sorry for my very bad english ^-^
(It happens on all browsers :D )
Okay, let's go... I got the same problem. But it's happening, just because a wrong code in the archive that validates the security code.
You must to edit this archive. Go to PLUGINS » EDITOR. Select the "simple CAPTCHA" plugin to edit. You'll se a sidebar with a list of archives. Click on the last (simpleCAPTCHA/zrx_captcha.inc.php). Now, you have to find this part of the code:
Now, change this line, for the line below:
This line will make the wrong code comments, go directly to the trash. Well, I hope I´ve helped.
(This reply is also available in this topic: http://wordpress.org/support/topic/plugin-simple-captcha-fails-on-correction-of-captcha-code-duplicate-comment-detected )