We have 2 monthly subscription tiers that are part of a subscription group, and always have been part of this group. Both are configured with a 1 month trial introductory offer. According to the documentation for auto-renewable subscriptions,:
Users can subscribe to one subscription product per group at a time.
And yet several users have managed to start trials of both products in this group simultaneously, which converted to paid subscriptions after the expiration of the trials, and now are being concurrently billed by Apple for both. How do we completely prevent this from happening?