In-App buys Apple ID against server user

I am trying to implement a multi-platform app with in-app purchases. I use MKStoreKit and non consumable content. I would use server side user registration. The application can be used without registering any user, so it will be possible to download content without registering.

The problem arises because I do not process purchases, but Apple (using Apple ID).

• If I buy one content using userA, then log in as userB and try to buy the same content, a warning appears that resembles something like “You already bought this in the App Purchase, but it has not been downloaded.”, As I can Deal with it?

• If I buy any content without registering and then register a user, how can I send my purchases and associate them only with this user?

• If I buy some content using userA and then change my Apple ID, I will no longer own this content according to my Apple ID, how can I deal with such situations?

It seems a bit messy to deal with Apple IDs, registered and unregistered users ... It would be easier to buy consumable coins, but I'm afraid of rejecting Apple.

I really struggled with this ...

Thanks for any help.