Just now I answered a question I remembered answering before. However, I can't find the question I previously provided the same answer to. This might very well be because it was a long time ago and I probably wasn't the first to answer it.
However, I don't know what the best thing to do with questions like this one is. The problem is that the asker usually doesn't realize what his question actually is. Instead, he'll be asking help with his attempt to work around the thing he misunderstood, which means that while the real question is always the same, the actual asked question is different most of the time. As an answerer, if you know about the common misconception you can usually confirm it with a single question in the comments.
Now the issue is that one could think of it as a duplicate question. On the other hand, future users with the same problem won't know what they are facing either, so it might actually be good to have the same underlying question lying around under different "titles".
What's the correct thing to do?