Kirk Douglas was known as Isadore Demsky when he grew up in Amsterdam NY in the early 20th Century.
Apparently it was a popular male name for immigrants and first generation children in the early 20th Century. It was often shortened to "Izzy."
There was a social trend in America during the middle of the 20th Century to "anglicize" names. For example, I have uncles who changed their birth name in the early 50s from "Wozniak" to "Wagner." Even Izzy Demsky became Kirk Douglas when he grew up.
Let's take for example, the children in "the Godfather" books, The older children have "old country" names (Santino, Fredo) and the younger children have "new world" names (Michael, Connie.) It's almost as if the older kids "Americanize" the family when they go to school.
Anyhow, names are funny things when taken in aggregate.