Skillmon
First off: We don't discourage cross posts, but we encourage stating when something is a cross post.
If something is a cross post (with the link given), how do we handle it if there is already an answer on the other post? Do we still answer, possibly copying (parts of) the other answer? Do we just ignore the question because it was solved otherwise?
Obviously there might be questions which have more than one correct answer and in such cases adding a new answer that shows a different approach is obviously fine, imho, but what about questions that really only have a single correct answer?
Top Answer
samcarter
Even if a problem only has one correct solution, there can be reasons to post an additional answer. For example a new answer can have a more in-depth explanation of the solution. Personally my criterion would be "If I were on the other site with an already existing answer, would I bring enough to the table to post an additional answer?" If yes, I'd post a new answer here, possibly referencing the existing one from somewhere else if my answer is build upon it.
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Legally it would be fine to bring over information from existing answer if properly referenced. TopAnswers offers the ability to license the answer with a compatible license.
Personally, I would either ask the original answerer if they would mind if I'd post an answer here based on the information from theirs (or if they might even be inclined to post an answer here themselves?)* or I'd just ignore such questions.
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(*I've done this a couple of times for cross-posts between the French site and tex se and it works great, the French site got at least one new user :) )