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

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)