iTunes gift card already redeemed. The Gift Certificate or iTunes Card you entered has already been redeemed
Apple refused to refund or help me resolve the matter (I had two cards with the problem).
See this thread and see that this is affecting many other customers.
Feb 2013 Update:
After approaching BBC local radio and getting them on the case I am pleased to say that a representative from Apple issued me with a full refund/replacement gift card codes.
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