title: Apple In-app purchase transaction_id of one purchase changes sometimes
body: <p>Once I get a receipt from client and validate it by apple server, then I got a transaction id of the new purchase (it's an auto-subscription purchase) in this receipt. After a few days , I get a new receipt from <strong>the same client</strong> and validate it , strange things happen : the transaction id of that <strong>old purchase</strong> changes.</p>
  
  <p>I compare the data of this purchase in the old receipt with the data in the new receipt , the only thing changes is the transaction_id field ，the original_transaction_id, purchase_date, expires_date, <strong>web_order_line_item_id</strong> and other fields are exactly same.</p>
  
  <p>Than I check the database, I found about 1% transaction records have the same situation. And there is a trait, Most of their transaction id were <strong>increased or decreased by 1-2</strong> .</p>
  
  <p>I used to think the transaction id is the identifier of a purchase. Does anyone meet the same problem or know the reason?</p>