We have had problems with this in the past, both in the PO-A screen for copying open PO's and PO-D screen for copying closed PO's.  
The multiple copy (one after the other) in PO-A appears to be fixed.  I could copy one after another with no problems.  The problem I have just identified is with a copy in PO-A, which assigned the PO # 3920.  Exited out of PO-A and went to PO-D to copy a closed PO.  The PO was given the same number as the previously copied PO (3920) and although the lookup screen in PO-A showed the new vendor, the PO, when you clicked on that vendor, shows the first vendor in the header screen, and has a combination of items from the first and second copied PO's. If I copy another PO at this point in PO-D, I still geta new PO with the same PO# ( I now have 3 po's with the number 3920) and this po has parts from all  three po's.  If I delete the second or third copy of the erroneous PO, the line items disappear on the other ones.
If I add a PO  in the normal way in PO-A in between copying the first in PO-A and the second in PO-D, everything works.
This may be a problem with PO-D only.  I didn't try the reverse order.