Duplicating a document allows you to create a new document from an existing document. This means you don't need to clutter your Templates with unnecessary duplicates when you can simply make a copy of an existing document.
To duplicate a document - first, locate the document you want to duplicate and click on it so you're taken to the document view page.
Then click Options > Duplicate, confirm the popup and that's it.
You'll be taken to the document view of your new document and from here, you can make any changes required by clicking Edit Document > Edit Setup.