PlayFab is not a game publisher, and it is common for a player to create several accounts on PlayFab. Your game is published to Apple Store or Google Play Store, the purchase of the game is linked to Apple and Google developer account, instead of PlayFab. You cannot restrict PlayFab login according to external service unless you have an external agent server.
However, if it is free to play, but includes micro-transaction, or needs DLC to unlock full contents, you may implement “flags” for player accounts, it can be either an inventory item, a KVP in Player Read-Only Data, which cannot be changed unless certain purchase is successfully done, so that when updating statistics, or entering certain level, you may add additional verification steps to make sure this is a valid player.
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