CAPTCHA is used in several contexts in MediaWiki and its extension: creating an account, trying to log in after failing with a wrong password several times, saving an edit with suspicious links, etc.

Many times I saw users who wrote a correct CAPTCHA string, but wrote something else in the same form incorrectly. For example, the second password was different from the first one. In such a case the form is reloaded and must be completely refilled, including a new CAPTCHA.

If the first time around the CAPTCHA string was filled correctly, the humanity is already verified, so it must not be shown again.