Here's how I see it. There are a few situations.
- Exact Duplicate
If the questions are asking the same thing, and you see one question has a proper solution, then it would be better to use the duplicate flag rather than clone the answer.
When you flag, users with at least 3,000 reputation will see the flag in one of the review queues and vote to close it as a dupe. Once you reach 3k, you'll be able to directly close questions (they will still go into Review, as 5 votes are needed).
- Same Solution
There's been discussion about the "lazy dupe," where people close one question as a dupe of another, just because they have the same solution. It's not a true dupe, and other users are annoyed at this behavior.
If you find the solution to the question on another Ask Ubuntu Q&A that is not a dupe, then the best practice is usually to phrase the answer in your own words and include a link to the original. A link-only answer should be avoided, as those are generally flagged and deleted.
- Similar Solution
This one could probably go without saying, but I'll put it in for clarity's sake.
If you find an answer on AU that could apply to the question you're dealing with, but isn't an exact solution, then adapt the answer in your own words and include a link.
For example, my answer about remapping capslock is very closely adapted from a more general answer for a question about making
xmodmappermanent. The question isn't the same at all, but the answer follows the same theme as the question I was researching.