Raise a warning on adding duplicated resource

Now, `add_route` merges consequenced `add_get(path, handler)` and `add_post(path, handler)` calls but doesn't do it if a resource for other path is pushed in between calls.
Also, it doesn't merge the same resource but under different names.

Let's not change behavior but raise a user warning for such cases, most likely it is the result of silly mistake/misunderstanding.