To reproduce:
1. Open attached project
2. Build for iOS
3. Deploy the project to an iOS device
4. Press the Home button to put the app to the app drawer
5. Press this link on the iOS device ( itms-services://?action=purchaseIntent&bundleId=your_bundle_ID&productIdentifier=doubletesting )
6. Observe: the app opens and prompts a purchase. Log in and make the purchase and a second purchase also pops up

Expected results: only one purchase is prompted
Actual result: two purchases are prompted one after another

Reproduced on:
2017.4.0f1 2017.4.21f1 2018.3.7f1 2019.1.0b5 2019.2.0a6

IAP Package:
-2.0.6
-2.0.1

IAP from asset store:
-1.20.1

Device reproduced on:
iPhone 6 Plus iOS 12.1.4

Notes:
Unable to build with IAP package 2.0.0