##The following is just my opinion.
I've been using Ask Ubuntu for about 3 years, so by now, I kind of know which question is a duplicate.  
Basically, most of the questions that get duplicated, are asked **a lot**.

So after a while, you can tell.  
But usually, if I'm going to answer a question I do a few things.

- I start by looking at the related questions in the right column. The system is quite smart. Most of the times, the duplicate is there.
- If it's something that's quite common, where in my mind I would be sure that someone must have asked such question before, (for example, those p) I search quite a bit to find the duplicate.
- If neither of the above, I do a quick search. If I find a dupe, I flag it. If I don't and I know the answer, I answer it.