Sorry for the confusion but this is normal (for the time being). There are multiple order numbers for a single purchase. You were not double billed.*
When you complete a purchase, an email notification is sent to your email address containing an order number. This is specific to the payment provider (credit/debit card, PayPal®, etc.) you used. The order number listed in your Transaction History of your NC Account is specific to our NCSOFT system, so it is different than the order number in your email. Our Customer Support Team can use any of the order numbers you provide to locate purchases.
We are working to have only a single order number presented in all locations to avoid confusion. Until then, the order number in your purchase notification email and the order number in your NC Account Transaction History will be different.
* If you do see multiple pending charges on your statement/account, please contact Support and we can look into it.