I answered a question yesterday; I didn't realise another question already dealt with the same subject. After having the answer accepted, I was asked if I could add the answer to the other question, as the question I answered was likely to be closed as a duplicate. I feel a bit uneasy about having the same answer for two questions (seems like two-bites-at-the-same-cherry). So, I have two questions about this; would it have been better simply to provide a link to the first answer, via comments or another answer?; do duplicates get closed, making it necessary to copy answers over in full?
When closing as a duplicate, the closed question is kept as a link to help people find the info, so if you know of a dupe, you can vote to close as a duplicate, or flag for a mod to do it if you don't have enough rep, and you can always add a comment with a link to your answer on the other question (voting to close as dupe should also add a comment for you)
If two duplicate questions both have good answers another option is to merge the questions.