On Stack Exchange, if you login with your email address and password and then get captcha tested, you have to provide your credentials a second time after passing the captcha.
I've been trying to figure out what the possible security advantage is to this. It seems to me that by authenticating in the first instance, I've proven that I have possession of the credentials, and that by passing the captcha I've proven that I'm a human. These seem like independent tests to me.
Is there a security advantage and if so what is it?