How much is "duplicate content" penalised in reality? How many times have you searched for an error message or technical issue and got a link to StackOverflow,

How much is "duplicate content" penalised in reality? How many times have you searched for an error message or technical issue and got a link to StackOverflow, and also on the front page some ad-laden site that's just a direct scrape of the exact same SO page? It's a common occurrence for me.