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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 stop(self):
 """Mark the stop of the interval. Calling stop on an already stopped interval has no effect. An interval can only be stopped once. :returns: the duration if the interval is truely stopped otherwise ``False``. """

        if self._start_instant is None:
            raise IntervalException("Attempt to stop an interval that has not started.")
        if self._stop_instant is None:
            self._stop_instant = instant()
            self._duration = int((self._stop_instant - self._start_instant) * 1000)
            return self._duration
        return False