Not at all. I have answered questions that have already selected most helpful before. I am on here to share my knowledge and try my best to help people. If I think my answer will be helpful I answer. the ratings and rankings on here make no difference to me. I just hope my answers help people who really are looking for help.
I don't care if a "Most Helpful" has been selected or not. I wanna help the user if I have a solution or opinion. I'm not just aiming at getting the reward. I enjoy helping others because it makes me feel happy about myself.
It does discourage me, especially if they select MHO with only one or two answers in a short time. They should wait for a few hours or days even, to select the best answer. I will always comment, tho, if I have something to advise.