how do we prevent users from registering duplicate for the app when they change apple store account to upgrade old package of previous apple store account to new package under new apple store account

To achieve this, you'll want to authenticate your users and have them log into an account for your app so you can associate the users between the two Apple App Store accounts. Then you can use the customerInfo object and check entitlements see the user already owns the subscription, you can prevent them from buying it by a warning pop-up, not showing that product, disabling the paywall, etc.
Please see our docs here on how to check subscription status: https://www.revenuecat.com/docs/customer-info
Please see our docs here on how we identify users: https://www.revenuecat.com/docs/user-ids