SAP CRM WebClient UI - Why use component usage to itself

The framework doesn’t allow to use the same view or window more than once in the repository browser. But in you example the view is assigned to a window and to a viewset. In order to do this, the view BP_ADDR/ContactWorkAddrEF is assigned to the window:

and then this window is used via a component usage in this viewset:

That means which each usage definition you can again use a window and assign it within the repository editor to another object. With every usage definition, a new instance of the component controller is created. If you want to avoid this, you can use the reference statement:

So BP_ADDR usage creates a new instance of this component, but with usage Dupl, you can use the windows again, but with the same controller instances as the BP_ADDR usages.