'Merge subtitle items for one language. Items with duplicated URLs
will be dropped.'

@staticmethod
def _merge_subtitle_items(subtitle_list1, subtitle_list2):

    list1_urls = set([item[u'url'] for item in subtitle_list1])
    ret = list(subtitle_list1)
    ret.extend([item for item in subtitle_list2 if (item[u'url'] not in list1_urls)])
    return ret
