We are having problems when using the same purchase order number on multiple sales orders.  We have installed a patch that was supposed to stop the system from warning us of duplicated purchase order numbers.  However, the patch didn't completely work.  The system will warn us that we have used the purchase order number before, and allow us to proceed through the header screen.  But it will not allow us to enter any line items.  Of course then it notes that there is nothing on the order and asks if we want to close the order.  Answering "No" sends us out of SO-A and back to the sales general screen.  The only way we have found to work around the problem is to leave the purchase order number blank until we enter and save the entire order.  Then we can go back into the order and edit it to include the purchase order number.  However, we don't know that the number is a duplicate until we try it the first time, so we lose the order the first time we enter it.  And we also have to remember to go back into the order and edit it after saving it the first time.  
This problem has been reported to DBA, but they do not seem to be in any rush to fix it.
Does anyone here have a fix for this problem?