question

Correct - Google does not provide a "restore" receipt which provides a fresh receipt for all in-app purchases, unlike Apple. They also don't have a requirement around restoring purchases, though titles should definitely consider implementing this. The way our system is designed, you pass receipts to us, to indicate purchases by the player. What we do is check that the receipt has never been used before, and grant the item if everything checks out (receipt hasn't been used, it's actually an item in your catalog, etc.). But that means that if the same receipt is sent to us, we won't grant the item again. This is specifically to prevent cheating - otherwise, the player could get the item over and over again by using the same receipt.

Your best approach is to actually get the player back to their original account by using the Link... API calls to add other authentication methods to it. Incentivize the player to add their Google Account, Facebook, or Steam, and you can then use that as the login mechanism if they lose their device and have to get a new one - just make sure part of the first-time experience in your game is to ask the user if they already have an account.

Unfortunately, until Google provides a similar system to the Apple "restore" receipt, the alternative is to re-use the certificate info to grant the items to the player, which would then create an easy exploit route for players who want to cheat the system and get the items multiple times.