Select Documents → Processing under Navigation and select the processing document type required. (Example: Customer Invoices)
A list view will open up on right side of screen displaying all current documents in the system.
Double click to open the required document.
On top tool bar select the Clone Document 
icon or click Processing on top menu and select Clone Document.
Changes can be made to the cloned document.
Save using the icon or CTRL + S or CTRL + Enter to save and close.