My understanding is that "duh" is an imitation of the sound a person with a speech impediment would make, which alludes to the notion that only a mentally challenged person would utter such an obvious banality aloud.
Russian doesn't have any widespread idiomatic construct for that particular nuance. There are many sarcastic responses to stating the obvious: да ну?, ещё бы!, а то!, да ты что? among others, but they lack the derision of the English one.
One way to convey that meaning would be an expression that used to be mildly popular in the late '90s:
— Чтобы жить одному, нужны деньги. — (your friend's name here) умный!
...uttered with slow nasal voice, as if you had cold and had a stuffy nose. This, too, is intended to be an imitation of a mentally challenged person.