PLanned order generated irrespective of open PO

Hi,

I have a purchase order for a material with proc type as F. After MRP run,a planned order is created for the same material and as a result the open PO is being set with an exception 15.(Postponing)

Why does not the system consider my open purchase order and why is it giving a planned order after MRP run..

What should I do to restrict this planned order creation if there are open purchase orders?

Please help me out