To Duplicate an Order Line item, from the maintain order screen, highlight the line item you wish to duplicate and then click on the Duplicate Order Line button

This will create a duplicate of the line item you had highlighted. You can now double click on the duplicate line item to edit the information for this particular line item. All the fields are duplicated over so you will need to adjust the information on the Line item screen. 

The purpose of the duplicate order line is if you have two of the same product but need to split it into two separate line items because the engraving setup may be different or possibly one is blank, or just about any other reason you can think of.