title: Google Play Billing: identifying duplicate transactions
body: <p>Is there any way to see if an in-app transaction concerns with an item already owned by a user? Our reports get a bunch of duplicate transactions due to people re-purchasing stuff (after removing the app or switching user accounts) – they don't get charged on Google side for purchasing the unconsumed items multiple times and we need a way to know that.</p>
  
  <p>We tried the following ways:</p>
  
  <ol>
  <li><code>OrderId</code> field from purchase receipt – seems to be unique every time,
  even if the item is already owned. </li>
  <li><code>PurchaseToken</code> – despite having a
  number of formats, it seems to be the same for transactions that
  involve purchasing the same item by the same person, <strong>however</strong> a
  few of our users still get unique purchase tokens every time they
  "purchase".</li>
  </ol>
  
  <p>Is there any way to tell if in-app purchase of this product has already happened in the past (like referring to <code>transaction_id</code> field from original iTunes receipt in Apple ecosystem)?</p>