The but tends to imply that there's no way to avoid ones destiny, it can't be changed.

"Every man has one destiny."
Well yea but it can be what you choose it to be.

"Every man has but one destiny."
Stop kidding yourself. You aren't a lawyer you're a mafioso. No matter what you try and do there's no way to change this pre determined fate. Michael wanted nothing to do with the family business, but his nature wouldn't allow it (for example).

"It's A Sicilian message. It means Luca Brasi sleeps with the fishes."