docs: def sum_of_reoccurring_values(x):
      """
      Returns the sum of all values, that are present in the time series
      more than once.
  
      :param x: the time series to calculate the feature of
      :type x: numpy.ndarray
      :return: the value of this feature
      :return type: float
      """
      unique, counts = np.unique(x, return_counts=True)
      counts[counts < 2] = 0
      counts[counts > 1] = 1
      return np.sum(counts * unique)