We Need a setup for MRP to PO Requisition.?

Can Any one help to done setup like that...

Suppose we Run the MRP on 01-jul-2014 and plan need date is 31-jul-2014 we get the requirement for Item X qty is 100 and Sugg Start Date 31-jul-2014

we release PO Requistion for that and generate PO or Item X with 100 Qty.

Now Again we RUN Mrp for next month in advance now MRP run date is  01-jul-2014 and plan need date is 30-AUG-2014, we also select Net Purchasing Flag. But we want next month MRP not consider Previous open purchase Order.

Because we need both the Requirement 100 qty for this month and 100 more for next month