Sometimes multiple questions can relate to the same topic from different angles. I've considered linking the questions, but sometimes the angles are so different that the answer to one question can include irrelevant information in relation to the other, require additional information or other editing. In such cases, would it be appropriate to "cross post" parts of an answer multiple questions and edit the answer to fit the new angle, or is this considered inappropriate.
Post the exact same answer to multiple questions is generally bad. If the answers have partial overlap and you're tailoring them to fit each particular question, go for it
WRT your example, I would have linked to the original with a brief explanation of why. This makes things easier to maintain if you or someone else decides to edit something at some point. Sometimes this might require you to flesh out and edit the original answer, which may produce a better overall result.
On the other hand, some people might consider an answer that just references another "lazy", and I certainly don't see anything wrong with what you've done in that particular case (it is a great answer!).
The point that "many causes can have the same effect", from the link in Mr. Mrozek's answer, perhaps applies better the other way around (many effects can have the same cause). These aren't duplicate questions, but the answers do converge.