He did not pronounce his name Doo-byoook, but it’s unlikely he cares, having been dead for a couple centuries (he died in 1810, in case you care). Julien Dubuque, seeing as how you’re that curious. There was an actual Dubuque who was actually French, after which the city was named.
Otherwise when you go there you’ll be constantly checking yourself before you ask a local resident, “So how do you find life in…your city?” (It’s a serious question, too, along the lines of David Bowie’s plaintive query “Is there life on Mars?”) Say “Doo-byoook” rather than “Doo-boook” or whatever other weird sounds the French-derived place name causes you to make. First thing you have to know about Dubuque is how to pronounce it.