Ensure that handler is not registered.

def register(self, *handlers, **params):
        """ Ensure that handler is not registered. """
        for handler in handlers:

            if issubclass(handler, PWModel):
                handler = type(
                    handler._meta.db_table.title() + 'Admin',
                    (PWAdminHandler,), dict(model=handler, **params))
                self.app.register(handler)
                continue

            name = handler.name.lower()
            self.handlers[name] = handler