Startselect is an authorized reseller of all the publishers listed on the Startselect platform.
Because of this, it is not possible that a code has already been used prior to receiving your order by e-mail.
If you do get the message that the code has already been redeemed, it is likely that a technical error took place while redeeming the code. In that case, it is best to log in again with your account.
This will ensure that the balance on your account will be retrieved again.