def replace_double_hyphen(value):
    """Replace ``--`` (double) with ``-`` (single).

    Args:
        value (str): string which have ``--``.

    Returns:
        str (``--`` is replaced with ``-``.)

    Examples:

        >>> val = "123--456"
        >>> replace_double_hyphen(val)
        "123-456"

    """
    value = str(value).replace("--", "-")
    return value