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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 add_callback(self, callback):
 """ Add a callback function to the listener. The callback function will be called for each indication this listener receives from any WBEM server. If the callback function is already known to the listener, it will not be added. Parameters: callback (:func:`~pywbem.callback_interface`):
 Callable that is being called for each CIM indication that is received while the listener threads are running. """

        if callback not in self._callbacks:
            self._callbacks.append(callback)