One of the episodes of Kevin Stroud’s History of English podcast mentions that a lot of compound words came about in middle english. One of them was “alone”. Kevin expected that his audience would be unaware that “alone” was a compound word. He was correct in my case.

Alone was a contraction of “all one” as in “all by oneself”. An added tidbit is that this preserves the old pronunciation of “one” which sounded more like the current “own”. Instead of pronouncing it as “a lone” like we do now, they’d’ve said it like “all own”.

This also means that “all alone” is equivalent to “all all one” but most folks aren’t too worried about it.