The following page shows how to copy an invoice using invoicefu. Foremost, you need to sign in the application. Once logged, click the 'INVOICES' button of the header menu to access your invoice's list. By clicking the duplicated invoice icon shown below, you'll be redirected to the invoice creation page. The new invoice will already be filled with data as it's a copy. You can choose to add content, modify content or just create the invoice wihtout modifying any content. To create the invoice, click on the 'Create Invoice' button located at the bottom right corner of the formulary.
Note: the duplicated invoice icon is also accessible from the invoice detailed view.