Thank you very much for helping me out in this problem.Still I haven't got the solution.
I tested the above mentioned configuration option by putting the Check Purchase Order Number field to 'A' in the transaction VOV8. But while creating sales orders, it is giving only information message. Is there any other way, to restrict the Duplicate PO value through configuration setup.
can you also explain how to use Enqueue and Dequeue FMs in the inbound posting function module.