I had this problem before and to fix this, Just make sure :
My problem (before) :
I had class : Core, Router, Permissions and Render
Core include's the Router class, Router then calls Permissions class, then Router __destruct calls the Render class and the error "Cannot declare class because the name is already in use" appeared.
Solution :
I added __destruct on Permission class and the __destruct was empty and it's fixed...