There should usually not be a need to post duplicate content on this website.
When you find two different questions which indeed have the same answer, then these two questions are duplicates. Instead of copy&pasting an answer you already posted on a different question, mark the question as a duplicate of that question.
When the question isn't a duplicate, then you should modify your answer so it better fits the different scope of the new question. Alternatively, if you believe that what you wrote somewhere else is relevant to the question even though it is not a full answer, you can always reference your previous answers by linking to them. Example:
Q: Can the president of Arstotzka fire the minister of defense because he was caught urinating in public?
A: As I wrote in this answer about firing the minister of agriculture for jaywalking, the president of Arstotzka can fire any minister for gross misconduct. However, according to Article 1345b (3) of the Constitution of Arstotzka, public urination is specifically exempt.