Misleading retitle
I think they renamed it "a fistful of dynamite" just to borrow from Leone's other famous westerns. But this is very different story material.
I think they should keep the "duck, you sucker" title. When a director is making something different, the marketing shouldn't confuse the audience in the hope of getting them to watch it by tricking people thinking it's another western. Because it's not.
This film is really its own thing.
Edit:
I just found out not even "duck, you sucker" is its original title. It's "Giù la testa". But this film does need an English title. From reading the posts, I found people said Leone liked "Once Upon A Time... The Revolution". It's probably much better.