I believe some [duplicate]/[closed] posts (those have up-voted/accepted answers) are useful. So can I edit such Question/Answers to improve the POST?
Answer is both yes and no.
It is valid to edit closed questions to narrow it down to it's on topic part, or to highlight differences enough for it to no longer be a duplicate. And it is valid to edit answers to closed questions, if it would be acceptable edit to answer to open question in the first place.
It is not valid to change meaning of a post. If you would need to put something OP never intended into a question to pull it out of "closed as duplicate" state, then it is not a valid edit. If you are OP of a question, then edit that invalidates already posted answers is not acceptable and may be rolled back.
If closed as duplicate question has a good answer that is not already mentioned under original one, you can flag that answer, or duplicate question under which it is posted, using
other (needs ♦ moderator attention)
and ask for question merge. It will move answers to the open question, saving them and exposing them to new readers properly. Merging is not always an option, though:
Keep in mind that merging two questions is done in specific cases; for example, if the one of the questions contains a detail that is mentioned in one of the answers, but not in the other question, then merging doesn't make sense. – kiamlaluno♦
So if questions contains different details, and answers actually use that details, edit that will show how they are different, followed by reopen vote (or moderator attention flag if you can't vote) may be a better approach.