Right-click on the source document to reveal the context menu.
Leave the reference unchecked to only duplicate the document.
Linked documents will be balanced depending on their balance type. For instance, if you create a credit note (credit) and link it with an invoice (debit), the invoice will be balanced with the newly created credit note.
Therefore, if you duplicate a document, for instance, you might not want to link the new document with the source document.
Uncheck line items you don't want to include to the new document.