Fonzer: Still didn't appear in my account maybe someone else got it as when i found this post it said posted now and i redeemed it but maybe someone was faster by a second.
Cavalary: If it said you successfully redeemed it, that can't be the case. Besides, once a code is entered, it's blocked until the redemption process is either canceled or times out, so it shouldn't be possible for multiple users to even try to redeem simultaneously.
What Harako__ said. The heavy load caused by massive redeems of Amazon Prime keys caused GOG to change the system a couple of months ago.
Right now, if the key is valid, it shows the "redeemed" message - with the load of validity-checking probably spread over multiple servers - but internally all redeem attempts go into a single queue, which are then processed sequentially, and only the first redeem attempt for each key will actually be added to the redeemer's account, with the others silently discarded.
(I suspect that before, there were also multiple servers, but they'd then synchronize / bottleneck to that underlying queue to make certain only one key was redeemed, and that caused the massive slowdowns.)