A number of people who have qualified for a free upgrade to OS X Mountain Lion through Apple's up-to-date program have found that after applying and receiving their code, they cannot use it to download the new software. When they enter their code in the store, an error pops up that claims the code was already used.
This error is rooted in the problem in which Apple prematurely opened its up-to-date program earlier this week. The up-to-date program was live for a few hours before Apple took it down, and during that time the program issued a number of redemption codes to those who had applied.
Apparently when Apple took the program offline, its code generation routines were reset, so when it officially went live this morning, a number of people who applied were given duplicate redemption codes. If the prior released codes have been entered in the App Store (which they likely have), then the newly issued ones would no longer work.
If you have this problem and cannot get your free update to Mountain Lion, then for now, wait. Apple is apparently aware of the issue and will be sending out new redemption codes to affected users by e-mail, which should happen in the next 24 hours.