I never knew A Fistful of Dynamite was also known as Duck You sucker! Truly you do learn something new every day
Amusingly, Leone was convinced that "Duck You Sucker!" was a common American expression, and refused to listen to anyone who told him otherwise. Duck You Sucker! is Leone's chosen title, and the one that was used in the original U.S. release (changed to a Fistful of Dynamite on re-release once the movie did poorly). .I think the Fistful title was used in Europe though, along with Once Upon a Time in the Revolution.
The restored dvd release uses the Duck title, at least here in the U.S.