I think that duplicates can be usefulcan be useful for various reasons. In particular, they might help to find a question if a user searches for different keywords. Similarly, they might appear in autogenerated lists of questions such as "Questions that may already have your answer" shown when you are posting a question or "Related" shown in the sidebar.
The titles are especially important for the purposes I mentioned above. An informative titleAn informative title can help with searching, improves chance that relevant posts are shown in autogenerated lists, and also makes easier identifying similar questions in these lists and in search results. So I believe that improving the title of a post, even if it is a duplicate, is useful for the site as a whole. (And perhaps also other improvements of such posts can be useful.)
But there is one downside to this. If I understand correctly the way how reviews work, such edit might put a post into the reopen review queueput a post into the reopen review queue. So even if I think that the post is correctly closed, by an edit I inadvertently might have started reevaluation whether the post is indeed a duplicate. (Fortunately, a question will only be enqueued once per closure via editing.)
Is it ok to edit such posts despite the fact, that they might get into reopen review queue? Or is it better to avoid such edits?