Duplicating the invoice becomes relevant when you need to invoice the client for the same amount within the same budget item.
To duplicate the invoice, use the Action button and select "Duplicate".
For instance, if you have a single budget item for the entire year and are invoicing the client monthly or quarterly, this feature streamlines the process. In the 2nd step of invoicing, the identical budget items with the same amounts are automatically retained. They are also automatically generated for the 3rd step of invoicing.
Note: If a budget item has already been invoiced up to 100%, it will be hidden by default in the second step of the duplicated invoice, since fully invoiced items don't display there automatically. To bring it back into view, check the "Don't show already invoiced items" checkbox in the top right corner of the second invoicing step.