Herzog said to be the same as duke
Current usage
Herzog is the root of many words of the same meaning in other Germanic languages, including Danish hertug, Dutch and Afrikaans hertog, Icelandic hertogi, Luxembourgish Herzog, Norwegian hertug, and Swedish hertig. 
Herzog was borrowed into other European language families with the chief meaning of the word being 'duke,' for example, by Balto-Slavic languages such as Belarusian hiercah (герцаг), the Eastern Herzegovinian dialects herceg (херцег; e.g. Herzegovina), Bulgarian khertsog (херцог), Latvian hercogs, Lithuanian hercogas, and Russian gertsog (герцог); by Finno-Ugric languages such as Estonian hertsog, Finnish herttua, and Hungarian herceg; and by Kartvelian languages such as Georgian herts’ogi (ჰერცოგი). 
The semantic equivalent of Herzog in Slavic languages is voivode or воевода, where voi- means 'army' and -vode means 'to lead' or 'to guide.' Both the Germanic and Slavic terms are used for place names within the Slavic-speaking world, as with the historical region of Herzegovina in modern Bosnia and Herzegovina and the autonomous province of Vojvodina in Serbia.
Herzog is not uncommon as a surname in German-speaking countries. The surname does not indicate an aristocratic origin, much like the family name King in English does not indicate royal ancestry.