A number of AppleInsider readers have reached out to express their frustration over the issue. In addition, numerous growing threads on the Apple Support Communities forums also document the problems.
Users are able to receive a redemption code from Mountain Lion through Apple's Up-to-Date program, but attempts to redeem it on the Mac App Store are met with the error: "This code has already been used. Each code may only be used once."
The issue is affecting users from all over the world, with users on Apple's site posting from the U.S., the U.K., Turkey, Australia, China and more.
It's currently unknown whether the error message is simply related to the influx of traffic on Apple's servers with users looking to download Mountain Lion, or whether incorrect codes were in fact sent out to customers.
Users posting on Apple's forums say they were told by company representatives that the issue should be resolved anywhere between 24 and 72 hours. Some others in various threads were apparently told that they will be sent a new redemption code from Apple to address the issue.
Customers who bought a new Mac on June 11 or later are eligible to receive a free upgrade to OS X Mountain Lion through the Mac App Store. Customers must request their copy of Mountain Lion from Apple's website before August 24, 2012.