As Apple mentioned in the App Purchasing Guide :
Shopping store development in a store
Your in-app shopping store should be attractive, intuitive, and easy to use. Follow these design guidelines to help you achieve this goal:
Provide a “Check inventory” button that allows users to check items that they may be missing or that they have already purchased on another device. Keep in mind that automatically initiating a restore of previous purchases (which will prompt the user about their password in the iTunes Store) may confuse your users.
In code, call restoreCompletedTransactions() to implement this.
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