I often add a comment to questions which is probably an answer. Usually I do this if there is already a very similar question or there is tonnes of information on the topic that a cursory search would find.
I don't really think it is constructive content to add to Stack Overflow; I only intend to help them with their issue, and hope the question gets closed. Should I always provide an answer when requested? What else is there to do? Usually, I just ignore them and think I will search for the duplicate when I have the time.