Subscription management with iTunes id (restrict the user from creating multiple subscription on same iTunes ID)
The user's iTunes ID differs from their Apple ID.  It is easy to 'share' your iTunes ID but hard to share your Apple ID.  

Your concern is a valid one but for a different reason.  No need to be concerned about two different users purchasing two different subscriptions.  But you could be concerned that two different users will share their iTunes ID and both benefit from a single purchase.  

One approach is to forget about it; it's a theft but not necessarily a loss of a real sale.  Another approach is to associate each purchase with the user's Apple ID using their iCloud key-value file.  Use that key-value file to implement your own restoreCompletedTransactions.  Do not allow a repurchase for free.