title: in-app purchase, multiple auto-renewing subscriptions having different durations
body: <p>A question regarding in-app purchase auto-renewing subscriptions:</p>
  
  <p>I'm unclear how to handle the case where a user purchases multiple subscriptions of differing durations.  It seems that Apple will recognize if the user has already purchased a given SKU (based on product-id) and prevent re-purchasing (my app gets a failed-transaction event in this case), but if I offer the same content using different product-ids having different durations, then the user can potentially purchase twice or more.  </p>
  
  <p>Even if I hide the additional purchase options once the user has subscribed once, they could potentially buy another subscription on an alternate device, and that purchase would/could ultimately be restored to the other device already having a subscription.</p>
  
  <p>Am I supposed to track purchased subscriptions by transaction-ID and product-ID, and calculate an ultimate expiration date?  That seems complicated.</p>