With Apple’s new OS X Mountain Lion update going live earlier this morning, many Mac owners have already navigated to the Mac App Store to upgrade their computers. However, it appears new Mac users are experiencing issues using Apple’s Up-to-Date program, seeing “this code has already been used” errors.

Update: According to MacRumors, AppleCare representatives will be re-issuing codes to those affected as soon as possible, but it may take up to one business day for it to be sent.

Mac owners that wish to upgrade via the Up-to-Date program are first asked to enter the serial number of their Mac computer and then are asked to wait while Apple verifies the status of their device. Once approved, customers are given a unique code to use on the Mac App Store that will initiate the download.

However, scores of Apple users are taking to Twitter to complain that their new code is being prompted with a “This code has already been used” message:

Screen Shot 2012 07 25 at 14.30.34 New Mac owners experiencing Mountain Lion upgrade issues, receive this code has already been used error

To clarify, the error is only affecting users that are utilising Apple’s Up-to-Date program and while other Lion users are seeing errors when they try to access the Mac App Store and download the update, they are not experiencing the code redemption bug noted above.

It isn’t known whether this is a temporary bug, caused by the huge amount of traffic passing through Apple’s servers, or the unique keys will show as ‘used’ from now on.

We are investigating and will update this article once we know more.

[Image Credit: CaliforniaDFG]