In App Purchase - when trying to buy consumable product again - this in-app purchase has already been bought

This happens if you haven't marked the transaction for the original purchase as finished, which you should do in your - (void)paymentQueue:(SKPaymentQueue*)queue updatedTransactions:(NSArray*)transactions method after you've successfully processed the purchase.

The method you need to call is [[SKPaymentQueue defaultQueue] finishTransaction:transaction].