Each time a customer browse the Order received (thankyou) page, your function code is executed… So for example, a customer can keep the Order received (thankyou) URL related to one of his orders, and browse it again another day, later.

So to avoid your issue you can:

• force the code to be executed only once per order,
• avoid orders with a creation date older than 3 days (for example).

Try the following replacement code:

add_action('woocommerce_thankyou', 'send_order_to_external_url', 10, 1);
    
function send_order_to_external_url($order_id) {
    if (!$order_id) {
        return;
    }

    // Get the order details
    $order = wc_get_order($order_id);

    // Exit if thankyou page has been already viewed
    if ( $order->get_meta('_thankyou_viewed') ) {
        return;
    }

    // Exit if the order creation date is older than "3 days"
    if ( strtotime($order->get_date_created()) < strtotime('- 3 days') ) {
        return;
    }

    // Prepare the data to send
    $data = array(
        'order_id'      => $order_id,
        'order_total'   => $order->get_total(),
        'billing_email' => $order->get_billing_email(),
        'billing_phone' => $order->get_billing_phone(),
        // Add other necessary order details here
    );

    // Convert data to JSON
    $json_data = json_encode($data);

    // The endpoint URL
    $url = 'https://example.com/your-endpoint';

    // Send the data via wp_remote_post
    $response = wp_remote_post($url, array(
        'method'    => 'POST',
        'body'      => $json_data,
        'headers'   => array(
            'Content-Type' => 'application/json',
        ),
    ));

    // Check for errors (optional)
    if (is_wp_error($response)) {
        $error_message = $response->get_error_message();
        error_log('Error sending order data: ' . $error_message);
    }

    // Tag the order as "thankyou" viewed (add custom metadata)
    $order->update_meta_data('_thankyou_viewed', true);
    $order->save();
}

It should work as before, but avoiding the issue you describe.

You can change easily the allowed number of days related to the date creation.