Force wp-cassify to match on user_email

umw-cassify.php

<?php

add_filter(                                                                     
    'wp_cassify_custom_parsing_cas_xml_response',
    function( $response, $user_data ) {
        // If a user already exists by this cas_user_id, no need to continue.
        if ( username_exists( $user_data['cas_user_id'] ) ) {
            return $user_data;
        }

        // If there's an email match, override the cas_user_id.
        $email_match_user = get_user_by( 'email', $user_data['email'] );
        if ( $email_match_user ) {
            $user_data['cas_user_id'] = $email_match_user->user_login;
        }

        return $user_data;
    },
    10,
    2
);