add-copy-to-customer-email-woocommerce.php

<?php

add_filter( 'woocommerce_email_recipient_customer_completed_order', 'your_email_recipient_filter_function', 10, 2);

function your_email_recipient_filter_function($recipient, $object) {
    $recipient = $recipient . ', me@myemail.com';
    return $recipient;
}

add_filter( 'woocommerce_email_headers', 'mycustom_headers_filter_function', 10, 2);

function mycustom_headers_filter_function( $headers, $object ) {
    if ($object == 'customer_completed_order') {
        $headers .= 'BCC: My name <my@email.com>' . "\r\n";
    }

    return $headers;
}