JogsterXL: I believe it locks the key for a short amount of time before it can be redeemed again.
So for instance if I drop a key here and someone were to enter it at the redeem page and see what it is but choose not to redeem it, the key would be locked for a few minutes. If someone else comes along and tries it shortly after it being locked, they would get the invalid key message. Then a few minutes later, after the lock period, if they try it again, they could redeem it.
Hope that clarifies things a little for you.
^ This was more first thought too. I'd give it some time before trying again.