def METHOD_NAME(used_mountpoints, new_mountpoint, old_mountpoint=None):
    """ Kickstart mode; check for duplicate mountpoints
        :param used_mountpoints: list of mountpoints currently in use
        :type used_mountpoints: list of str
        :param new_mountpoint: mountpoint selected by user
        :type new_mountpoint: str
        :param old_mountpoint: mountpoint previously set for this device (optional)
        :type old_mountpoint: str
        :returns: mountpoint validity and error msg
        :rtype: (bool, str or None)
    """
    if not new_mountpoint:
        return (True, None)
    if not os.path.isabs(new_mountpoint):
        msg = _("\"{0}\" is not a valid mountpoint.").format(new_mountpoint)
        return (False, msg)
    if new_mountpoint in used_mountpoints:
        if new_mountpoint == old_mountpoint:
            return (True, None)
        else:
            msg = _("Selected mountpoint \"{0}\" is already set for another device.").format(new_mountpoint)
            return (False, msg)
    else:
        return (True, None)