For that I think you need a user database on your server, and a login system. Every time the user logs in from a device, the app would send the server list of items it has downloaded/installed, the server will cross check this against items they have purchased, and if there are items that are not downloaded to that device yet, it would tell them about it and give an option to get the content. That's my guess, I haven't implemented subscription based IAP yet. See the IAP Guide in the docs as well, it probably has some pointers on that.
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