Isn't the appropriate response to a duplicate question "close as duplicate" ?
The fact someone bothered to ask the question even though it's a dupe, probably means he is not aware of the dupe question's existence. The next person to come after him and ask the same question in similar wording will see that his question is a dupe before asking it - so asking a dupe question usually adds benefit to Stack Overflow / Meta.
I do not consider it a bad behaviour (you can call it a bit lazy, because a more or less through search before asking could have discovered the dupe... but lazy != bad. As I said, asking a dupe is actually beneficial to the community).
What are your thoughts on this?