We do iap receipt verification on server and we found out that there were some players trying to send us same receipts for different purchases, which means different transaction has same transaction id, is that possible that this is a bug on Apple when user tries to buy the same item in a short time, Apple will send an old receipt?
I got same receipt for two purchase for the same product.
The problem is that an original purchase, a restoreCompletedTransaction and a repurchase (for free) (all on the same device) will have the same transaction_id (and original_transaction_id!) in iOS 7 Style receipts. This wasn't true in iOS 6 Style receipts. If you are looking for uniqueness in a receipt you now need to include the receipt creation date (creation_date). If a receipt has the same creation_date and transaction_id then it is a copy (i.e. a fraudulent attempt to claim a second purchase)