Suppose someone needs to ask a perfectly legitimate question, but that same question already exists on StackOverflow. However, the other question does not have an answer yet. Of course users are always discouraged from creating duplicate questions, and eventually get closed/deleted/merged. What is this user expected to do to get an answer for that question?
My first thought was to create a bounty for it, but that wouldn't be fair. Just because someone else asked the same question and it's not answered yet, doesn't mean I should give up extra points to get an answer.
A more ideal solution would be to put some sort of flag on the original question which bumps it back into activity, and treats it somewhat as a new question. I know this can be done in some cases by just commenting on it, but users may not know this and we don't want to pollute comments either.
So, I propose a new flag users can click on existing questions "I have the same question", which when flagged, this question will appear as if it were new. I'm sure some rules would have to be in place, for example Question must be at least a month old, and of course Question does not yet have an accepted answer. Then, other users can see on each question "X users also have the same question".
Such a feature will bring attention to old questions which have not yet been answered, and which widely need an answer. You can already explicitly view unanswered questions, but this is sorted by number of votes, and votes don't always tell the whole story. There may be a really good question which has 50 up-votes, but only one person needs the answer. But at the same time, there may be another really good question with little to no votes, which applies to many different people. I myself have asked one or two questions which never got a proper answer, and are still unaccepted.
This feature would also be of good use if it always allowed any and every user to use it, because it would present a good understanding of how much that question actually helped. Someone may ask a question which gets closed for "too local", but then 20 people might flag it "I have the same question". This type of activity could trigger an automatic re-opening of the question, if this were the case.
The same feature can also be used for featured questions, where if a) question is at least a month old, b) question has no answers, and c) at least 5 people voted "I have the same question", then place this question in the Featured questions (which is only showing bounties currently, if I'm not mistaken). This way questions which really need an answer can show in the featured list along with bounties.
Just adding this one flag alone, even without all the extra automatic detection as described, could help tremendously in identifying whether questions are actually useful. It can be introduced just as a single link "I have the same question", and a display "X users have the same question" (if applicable), and that's it. Then, future updates to StackOverflow could include these extra detection techniques as I described.
Another use of this feature would be to detect automatic FAQ, because as I said above, votes don't always tell the whole story. Questions which have at least 20 "I have the same question" votes could be flagged as FAQ. As you can imagine, this single flag could open many doors in analysis of questions in all StackExchange sites.