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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def register_model_once(cls, ModelClass, **kwargs):
 """ Tweaked version of `AnyUrlField.register_model` that only registers the given model after checking that it is not already registered. """

        if cls._static_registry.get_for_model(ModelClass) is None:
            logger.warn("Model is already registered with {0}: '{1}'"
                        .format(cls, ModelClass))
        else:
            cls.register_model.register(ModelClass, **kwargs)