To duplicate a list, you'll first need to make sure that context menus have been enabled in your app settings. Then just right- or control-click the name of the list you would like to copy in the sidebar to access the context menu. There, you'll have the option to Duplicate list.
Pro tip: if you have lists that you often re-use, like a groceries or packing list, you can create a template. Each time you need to head to the airport or grocery store, just create a copy by selecting Duplicate list.