Purchase order number in sales order

Hi friend.

When we create sales order, evry time system ask for PO number, I know PO field is mandatory in customizing. My question is that every time what ever number even same number e.g. 1 we enter in PO field in sales order, system allow to save. I want po number should come automatically and if i have any po number, system should displau error message that this po number already exist. How this can be done.

Thanks

Dave