transaction_id issue

[Waqas Saeed]

my payment gateway does not return transaction_id in json response, What is the method to add the transaction_id, so there shouldn't be duplication if the callback URL is hit on the browser manually.  WHMCS has a simple flow, it does not check if the invoice is paid, or unpaid, if the callback file is directly access it might be possible to add the same payment again if the transaction is different or no transaction id.