ReceiptStoreOperations.list_receipts_since return messages more than once. And are duplicate messages

[rohitmp]

When the initial listen to scabbard events ws request is called.
Current Behavior: The method ReceiptStoreOperations.list_receipts_since, misses return messages more than once. And are duplicate messages
Expected: It should return only all the events only once.

expected 20 nos as per below attached log; but its returns 39 messages.

Attached is the

1. the transaction_receipt table values. (there are 20 rows) splinter-beta-transaction_receipt-table-values.txt
2. log file from a java ws client trying to list the scabbard events (transaction_receipts). (it returns 39 messages)
   scabbard_events_with_dulicates.txt