Bulk Fulfillment of Sales Orders via CSV Import Results to Failure with Message “You have an invalid sales order xxxx or the order is already closed." but Item Fulfillment still gets created.

Sales Order had been entered more than once in the CSV file. 

In the Processed Records page (Transactions > Sales > Fulfill Orders > Status > Complete link), click the Record column to Sort by Record/Sales Order #.  Look for the sales orders with Message = "You have an invalid sales order xxxx or the order is already closed." 

The line above will show the same sales order # as successfully completed with the resulting Item Fulfillment.