It seems like you're asking what to do when:
- an old question would be considered off-topic if posted now, but was never closed as off topic, and
- a new question is posted, that is a duplicate of it.
Close the new question as a duplicate of the old question, if it's really a duplicate. There's no reason for people not to benefit from content that already exists on our site.
That's the closest I can come to giving the "single answer" that Oli quite correctly says doesn't exist. Really, more information is needed, so if you are ever unsure about a particular question, you might consider discussing it with others in chat and/or posting a more narrowly tailored meta question about it.