In addition to the Documents → Copy document option on the context menu of the "Document list" screen, to create exact copies of the selected document(s) of a specific document type, you may also copy any of the documents to another document type.  

If you need to select multiple documents, press the Ctrl or Shift key and click on each document, which you need to copy.

Right-click and select Documents → Copy to → Invoices, Credit notes, Quotes, Purchases, Supplier returns or Orders option on the context menu. New documents for the selected document type will be created.

Once this is done, you may edit / post these documents.

Once a document is copied, the selected source document fro which it is copied, will be added to the third message line. 

You need to edit the first and second message line if required. 

Note the Document message (set in the Setup → Documents setup on the Setup ribbon) will also be added to the copied message of the selected source document.