WooCommerce action thankyou.php and functions.php

functions.php

/**
 * file: functions.php
 * Update meta data when someone's looking at their order thank you page
 */
function thank_you_page( $order_id ){
    $order = wc_get_order($order_id);
    $order->update_meta_data('_user_has_loaded_thank_you_page', 'yes');
    $order->save();
}
add_action( 'woocommerce_thankyou', __NAMESPACE__ . '\\thank_you_page', 10, 1);

/**
 * file: thankyou.php
 * Push to the dataLayer only when order meta data is not "yes"
 */
<?php if ('yes' !== get_post_meta($order->get_id(), '_user_has_loaded_thank_you_page', true)) { ?>
  <script>
    window.dataLayer = window.dataLayer || [];
    ........
  </script>
<?php } ?>