Hi,
the plugin works fine, but it's not displaying any error message when two different mail addresses are entered.
In your code there's the wpcf7_confirm_email_validation_filter function where you're using wpcf7_get_message( 'invalid_confirm_email' ) to get the error message. But 'invalid_confirm_email' seems to be undefined.
When I enter a string instead of the wpcf7_get_message function, the message is being displayed.
https://wordpress.org/plugins/contact-form-7-confirm-email-feild/