Allow duplicate transaction ids for payments which go from pending to…

… completed. When this happens the payment should not be flagged but processed like any other successful payment. This can happen for large amounts of money, currency conversions, transactions from certain countries, and other reasons at the discretion of Paypal.

paypal/standard/helpers.py

@@ -4,8 +4,19 @@
 
 
 def duplicate_txn_id(ipn_obj):
-    """Returns True if a record with this transaction id exists."""
-    return ipn_obj._default_manager.filter(txn_id=ipn_obj.txn_id).count() > 0
+    """Returns True if a record with this transaction id exists and it is not
+    a payment which has gone from pending to completed.
+    
+    """
+    query = ipn_obj._default_manager.filter(txn_id = ipn_obj.txn_id)
+
+    if ipn_obj.payment_status == "Completed":
+        # A payment that was pending and is now completed will have the same
+        # IPN transaction id, so don't flag them as duplicates because it
+        # means that the payment was finally successful!
+        query = query.exclude(payment_status = "Pending")
+
+    return query.count() > 0
 
 def make_secret(form_instance, secret_fields=None):
     """
@@ -46,4 +57,4 @@ def check_secret(form_instance, secret):
     """
     # @@@ add invoice & custom
     # secret_fields = ['business', 'item_name']
-    return make_secret(form_instance) == secret
+    return make_secret(form_instance) == secret