Contact Form 7 Thank You Message Appearing Twice

I’ve made a thread about this before, but it was closed. The issue still persists, even with latest Contact Form 7 installed.

The last user mentioned something about deregister in my functions.php file, but I don’t know what that is. I looked in functions and this is the only ‘register’ piece of code:

function wpcf7_register_post_types() {
	if ( class_exists( 'WPCF7_ContactForm' ) ) {
		WPCF7_ContactForm::register_post_type();
		return true;
	} else {
		return false;
	}

I’d love to have this solved! Please help. Thanks!