When attempting to post a PO, a duplicate PO error is returned "record already exists"
Someone may have created a PO Number that was a future PO number. When the system reaches that PO number, it returns a duplicate PO number error when attempting to post a new one.
This can be fixed in Purchase Orders -> PO Setup -> Options -> Documents Tab -> Documents Number -> Change Next Document Number (increase it by one) then click Save and Close.
See also Record Already Exists