Under the circumstance I am right now. I cannot purchase a second copy from the official website, because it linked my first purchase directly with my account, without a code redeem operation.
And now it simply does not even let me click on the items at the store anymore.
So I can only purchase a second copy if I create another account or use amazon for example. But amazon is not an option if I want to keep the gift a surprise, since the online pre-order code requires a US billing address, which I do not have, so I would have to contact my sister to use her amazon account. The other option, creating an account for my sister to use and then buying through it, also spoils the gift, since I will have to give her the task of creating such an account (because I don't want to mess anything up and because I obviously do not have access to her email for the account creation confirmation).