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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 instance(cls, public_keys_dir):
        '''Please avoid create multi instance'''
        if public_keys_dir in cls._authenticators:
            return cls._authenticators[public_keys_dir]
        new_instance = cls(public_keys_dir)
        cls._authenticators[public_keys_dir] = new_instance
        return new_instance