Trying to Add Requisition to Exisitng PO

We have an approved requisition that has the same Category, Description, Unit of Measure, and Unit Price as an existing PO Lines.  When we try to add it to the existing PO Line, it returns the following error:  "You cannot combine this line with another line because the Source Documents are different".  Does this mean the blanket needs to be referenced on the requisition?????