I think, regardless of google, bing, and other search engines, if you know the answer and you want to spend your time answering it, then answer it.
If the answer is that it is another stack overflow (i.e. duplicate) then say so and reference it.
I have many books on c++ programming. Much of the material is overlapping, but the presentation and extra information presented by each author provides a broader picture. I often find that I need to look at not one but several stack overflow articles with differing perspectives to really get an answer I can use. Stack overflow has the advantage of differing views on the same question, sometimes multiple times as it is asked with subtle variations. These help me to find answers that google may not.
Also, while entering a question in stack overflow is relatively simple, they may not find the answer so easily in google. Not everybody is a good googler. Google has amazing algorithms slicing and dicing the data. Stack overflow has a community of people who look through the new/open questions (yay). I think it is better to answer the question than make the value judgement that they should have just googled it and gotten the answer.
So my response to the question is: Give the user the link if you found it to be relevant. Expand and add to it if you can (adding to their potential understanding beyond just giving them a link). Leave the judgement about whether or not they should have found it by googling behind and just be helpful to the community.