Sometimes you may get a message from Audible saying that a code has already been redeemed. This is because our system has no way of staying in sync with Audible. So we do our best to keep them in sync, but it's not perfect. And it's up to the authors to update their codes to make sure they are accurate. Wait 3 minutes and you should be able to claim another code.
We at FreeAudiobookCodes.com don't have the ability to update codes, so please don't enter a support ticket. We don't have a way of helping with this issue.