"Your message was sent successfully." appearing twice

Might be a theme issue. Have you installed any other plugin for contact form apart from contact form7 ?

No, only Contact Form 7. This error seems to have only shown up with the latest version of CF7.

Anyone?

I have a same issue,
I have install the really simple captcha plugin also.
and there is div like <div class=”screen-reader-response” role=”alert”>
is extra came.

This is still an issue for me. Has anyone found a way to fix this yet?

Maybe you are using deregister for contact form style. Comment this in your functions.php and try again.