Invoices can be copied one at a time or more at once.
In order to copy a single invoice, click on the Copy option in the menu. 
When the invoice is copied, date of the new invoice is set to current date. Data that is copied from the original invoice:
- Basic information - partner, category, currency, TAX included, department, payment method.
- Invoice items - items and quantities are copied while the prices are set based on the current price in the price list.
Link to the copied document is visible at the bottom of the invoice Details/Edit page.
Bulk copy option
To copy more than one document at a time you can use Copy option from the bulk options menu. 
Select  the invoices to copy, and then click on the [COPY] button.