We are considering using Revenuecat for cross-platform support, but our service, is designed so that the same user can have multiple accounts, and the system does not keep track of which users have which accounts. We are told that cross-platform support requires the use of a custom App User ID to track users across platforms, but if our app's user_id is set to a custom App User ID, they will be treated as different users, and we will not be able to track users if the platform changes. How can we achieve cross-platform support while preventing duplicate purchases of the same subscription plan when the same user currently has multiple accounts? Would we need to build a separate system to have users decide on a parent user_id and integrate them even if they have multiple accounts, so that Revenuecat sees a one-to-one relationship?