It's kind of like you're just calling the asker stupid...

Saying "duh" in general bothers me because it's meant to mock those with mental disorders and there's nothing wrong with having a mental disability. I don't understand why people like to laugh at it.

^I agree.

Sorry, I had to :(

But seriously, some questions are so incredible stupid...

It's offensive and rude sometimes.

It just means that the person doesn't have enough education and vocabulary to say anything else, therefore makes them look like the moron.
Honestly though, I get really pissed off when people say that.

uh...duh!

It doesn't usually bother me, but seeing that as your only reply to everything is annoying.