How to duplicate render layer could be render layer duplicated? With all its properties and freestyle settings preserved ?

There is only button for adding, but newly added layer is not set.
At the top of your Blender window, select Cycles Render.
In the Screen-layout drop-down, select Compositing.
At the bottom of that panel, put a check next to Use Nodes.  

You can now branch the Render Layers Image output to several nodes.
You get to decide which nodes to use.  Just Add whichever ones you need,
then send the output from your nodes to Composite Image.