How do I duplicate an invoice?
Login and go to Sales. Filter or otherwise find the one you'd like to duplicate.
Click on the invoice and you'll see a nice representation of the final invoice that is ready to be sent/was sent to your customer.
Click on the More dropdown at the top right of the invoice, and select Duplicate.
You will be taken to the New Invoice screen, with a new unique invoice number and today's date filled out, but with everything else on the invoice the same.
Fill it in to your liking and then hit Create Invoice at the bottom to finalise the duplicated invoice.