If you don't have a lot of time but want to be helpful:

Comment on the question, leaving a link to the older question

Otherwise, I'd go for:

Quote that answer (or reword it, possibly making it more valuable) and flag older (more specific) question as a duplicate

Generally, links to other posts on Super User are not as prone to link rot as external links. It's quite reasonable to link to another answer, without repeating much. However, this is often an indicator of the question being a duplicate (in the sense of being answered over there, not being the same question). In that case, what we'll often do when we see these answers is 1) deleting the answer and 2) closing the question as a duplicate.

In the specific case you gave us in your post here, if we'll find that the current question is more general than the older one and answers to it would be equally applicable, we can reverse the typical duplicate direction.

Taking the content from one answer, extending or rewriting it (with proper attribution) is always fine, provided that you're not just blatantly ignoring to vote/flag to close the question for your own reputation gain.

If you don't have a lot of time but want to be helpful:

Comment on the question, leaving a link to the older question

Otherwise, I'd go for:

Quote that answer (or reword it, possibly making it more valuable) and flag older (more specific) question as a duplicate

Generally, links to other posts on Super User are not as prone to link rot as external links. It's quite reasonable to link to another answer, without repeating much. However, this is often an indicator of the question being a duplicate (in the sense of being answered over there, not being the same question). In that case, what we'll often do when we see these answers is 1) deleting the answer and 2) closing the question as a duplicate.

In the specific case you gave us in your post here, if we'll find that the current question is more general than the older one and answers to it would be equally applicable, we can reverse the typical duplicate direction.

Taking the content from one answer, extending or rewriting it (with proper attribution) is always fine, provided that you're not just blatantly ignoring to vote/flag to close the question for your own reputation gain.