I tried putting "Possible Duplicate" in the block quote, but my answer got converted to a comment here.
I am sure this question itself is a possible duplicate - I just can't find the one it is duplicating.
You can click the flag button (which is just right of the edit one), then choose the option "The question doesn't belong here".
You will then get to this screen:
All you have to do is enter the URL of the possible duplicate, click the Flag Question, and you are done!
First, if you know it is a duplicate, don't answer it. Any answer belongs on the duplicate question. And don't try to manually add duplicate info, once the question is close it will be added automatically.
To flag you first need to click the flag link:
Then select "it doesn't belong here or is duplicate"
Next select "duplicate"
And finally enter the duplicate info by selecting the question below or entering the exact link or question id in the text box