How to check if user has already downloaded non-consumable item Inapp purchase

I am providing non-consumable item in my application.

suppose user purchases that item and tries to install that application on another device with same id . If the user is logged in and already have purchased that item , i want to show some pop up and tell user that you have already purchased it so download it for free now.what happens right now is , when i click to purchase item again then is says that u have already purchased that item click to download.

Is there any way to get user login details and check if user has already made In-app purchase or not.

there is a problem when i login and try to buy already purchased item . two kinds of alert view appears time to time . i am using sandbox environment.

1. you've already purchased this but it hasn't been downloaded.Tap OK to download it now. this alert view has only OK button .when we press Ok control goes to - (void)failedTransaction:(SKPaymentTransaction *)transaction

2. you've already purchased this.Tap OK to download it again for free.this alert view has two button one is Cancel and another is OK . On pressing OK button control goes to - (void)completeTransaction:(SKPaymentTransaction *)transaction which is fine.

An idea why transaction is showing this peculiar behavior .

thanks