[Sandbox] Consumable products "already bought"

My app only offers consumable in-app purchase products.

I can successfully purchase a product once but when I try to purchase it again, a popup "This In-App purchase has already been bought. It will be restored for free." appears and the first transaction is restored: the payment queue delegate is notified that a new transaction (with the same identifier as the successful transaction) is updated to the purchased state. I never get a purchased transaction with a new identifier and hence I consider that the purchase failed.

Also, each time the app is put in foreground again or at startup, the payment queue delegate is notified that a transaction has been updated (as if it had never been finished). Even though the app properly finishes the transaction each time.

"This In-App purchase has already been bought. It will be restored for free."

This message indicates that you did not call SKPaymentQueue's finishTransaction: in your application. Calling finishTransaction: allows you to remove a transaction from the payment queue. It notifies the App Store that the application acknowledges the transaction so that the App Store can mark it as completed.

Be sure to follow Setting up the transaction observer for the payment queue.