In the document browser or tap on "Select" in the top right corner.
Select the document you want to duplicate.
Tap on the duplicate button from the bottom (which is the second button from the left)
A new copy has been created. See our How do I move a document? article if you want to move the newly created copy to a new location.
This function can be used if you want to edit the document, but still want to keep the original file.