Should a citation be included when answering a question regarding a specific source?
If it is a discussion board the audience are only the course instructor and the students of the course, whether or not you repeat the citation in your answer is not going to have any important or lasting effect. That said, while a very strict instructor might complain if you do not repeat the citation, I cannot imagine any way in which repeating the citation could have a negative effect. If you are in doubt, just cite it again.