Check duplicate entry

I have a form with a text field and i need to check if it exists any entry with the same value before sending the e-mail.

I try this:

add_action( ‘wpcf7_before_send_mail’, ‘check_rg’ );

function check_rg( $cf7 ){
	$args = array(  'post_type'  => 'inscricao','meta_query'  => array(array('value' => $cf7->posted_data["rg"])));
	$my_query = new WP_Query( $args );
	if( $my_query->have_posts() ) {
  		$cf7->skip_mail = true;
		echo "<script type='text/javascript'>alert('Duplicate Entry');</script>";
	}
	wp_reset_postdata();
}

It works, but after the alert, the data fields turn empty and the success message appears bellow.

Any idea to do it better way?

https://wordpress.org/plugins/contact-form-7/