Since the question is answered on another site, I shall take the advice in that answer and its comments.
TL;DR: Take the relevant parts of the answer from the other site and summarise/quote in an answer exactly as you would for any other internet reference.
For questions that are on-topic on both stacks, not asked by the same person, and already have a good answer on the other stack:
Link to the other question with a community wiki answer.
Also, it could be good to include the full text of the post in block quotes (using >) in case it gets deleted, and linking to the other author's profile on the other stack.
- Allows the question asker to get the information they are looking for
- Has no risk of receiving reputation for another's work
- Prevents any worries of plagiarism
This is the best compromise solution.
Well, as long as you quote and reference properly, I don't really see a problem with not making this community wiki. Afterall you usually get reputation for quoting and summarizing other sources from the internet anyway, so why not another SE answer. If the answer's good give that guy his rep, CW is IMHO counter to the workings of SE in those case. – Christian Rau
Personally, I don't have a problem with extending the "Duplicate" closure reason to other Stack Exchange sites, but as that isn't likely to happen, this seems to be a good solution which follows existing practice for information on non-SE sites.
This answer only applies to a question which, while answered elsewhere, is still on-topic on ELU. Where a question is answered elsewhere because the answer is where the question is on-topic, then the question should be migrated and treated as a normal duplicate. That may even happen between ELU and ELL [in either direction], as well as a question being on-topic in both places.