My opinion is that if you come across a question that you are able to answer, and you are not personally aware of any possible duplicate questions out there, then go ahead and answer the question.
As an answerer, you shouldn't feel obligated to search for duplicates of every question you come upon. That is something the asker should be doing, and to a certain extent you should trust that they have already done that. If there is a duplicate out there that you don't know, someone else will likely find it and mark it. You shouldn't feel bad if a question you answered ends up begin marked a duplicate, since you acted in good faith and were not aware of the other question. At that point you can even go read the answers on the other question and see how your ideas match up.
As you get more experienced here and start to read more and more questions, you will begin to see new questions that seem very familiar, or seem basic enough that you would be surprised if they hadn't been asked before. That's when I might go in search of a duplicate - only when I already suspect that one exists.