If you have sufficient reputation, you can vote to reopen.
If you don't, you can edit the question to help clarify why the proposed duplicate is not in fact a duplicate of the question. (While you can be explicit about this, you can also be implicit and simply emphasize or clarify the points that were confusing enough to result in the closure to begin with.) Usually there is some reason a question is closed as a duplicate. Even if it's not a duplicate, there is often some point that is vague, unclear, or simply buried too deep in the post for the readers to realize why the question isn't a duplicate. Editing the post to help clarify this, especially if you don't mention the duplicate, or the closure, explicitly, can really improve a question.
Both actions, either voting to reopen or editing, will put the question in the reopen review queue, which will give it enough attention by 3k+ users to determine if it should be reopened or not.
You don't need to make a meta post for every single question you see that you think should be reopened. You should generally reserve such attention for particularly controversial posts in which it's unclear if a question should or should not be reopened, or questions that are being repeatedly reopened and closed due to disagreement among users.