woocommerce_checkout_order_processed hook executing function twice - php

Question

I have attached a function to the woocommerce_checkout_order_processed hook:
//check if woocommerce is acive
if (in_array('woocommerce/woocommerce.php', apply_filters('active_plugins', get_option('active_plugins')))) {
add_action('woocommerce_checkout_order_processed', 'wc_on_place_order');
}
The wc_on_place_order function is to be executed after the user clicks on the PLACE ORDER button. However, it's so odd that the function is executed twice.
My wc_on_place_order function calls an external api written in C#:
function wc_on_place_order( $order_id ) {
global $wpdb;
// get order object and order details
$order = new WC_Order( $order_id );
// get product details
$items = $order->get_items();
//return $items;
$products = array();
foreach ($items as $item) {
array_push($products,
array('userid' => $order->user_id, 'descr' => $item['name'], 'amt' => (float)$item['line_total'])
);
}
//passing $products to external api using `curl_exec`
. . . .
//on successful call, the page should be showing an `alert`, however, it does not
// the handle response
if (strpos($response,'ERROR') !== false) {
print_r($response);
} else {
echo "<script type='text/javascript'>alert($response)</script>";
}
}
After debugging on C# API, I noticed that it the service is being called twice, thus, the checkout is being saved twice to the API database.
Is there something wrong with the wc_on_place_order function or is woocommerce_checkout_order_processed called twice when clicking the PLACE ORDER?
Interestingly, adding return $items after $items = $order->get_items() somehow, the C# api was only called once:
// get product details
$items = $order->get_items();
return $items; //this line
Why is that so?
One more question I would like to ask, is woocommerce_checkout_order_processed the right hook I should use? I have been searching the web for the correct hook to use and it seems that woocommerce_checkout_order_processed is used in the most post. I can't use the woocommerce_thankyou hook as it is also calling the API if I refresh the page.
Any idea will be really appreciated.
EDIT:
I used woocommerce_after_checkout_validation hook which fires after pre-validations on checkout. I can't remember though why woocommerce_checkout_order_processed is being fired twice but I just changed some kind of settings in WooCommerce options page. I can't remember which.
Useful Links from the Comments:
Visual Representation of the WooCommerce hooks
WordPress Action References

Answer 1

I always use the hook woocommerce_payment_complete This will fire as the name suggests after the order has been paid.
function order_payment_complete( $order_id ){
$order = wc_get_order( $order_id );
/* Insert your code */
}
add_action( 'woocommerce_payment_complete', 'order_payment_complete' );