“Repeat email” field

Custom solution:

add_action('um_submit_form_errors_hook_', 'my_um_check_email', 999, 1);

function my_um_check_email($args) {
    global $ultimatemember;
    if (isset($args['user_email_2']) && isset($args['user_email']) && $args['user_email_2'] != $args['user_email']) {
        $ultimatemember->form->add_error('user_email_2', 'Please repeat your email address.', 'my-key');
    }
}

From documentation: http://docs.ultimatemember.com/article/94-apply-custom-validation-to-a-field