A lot of easier questions get answered very quickly, by several people within the course of a few minutes.
These solutions are often redundant because the users were mutually unaware that others were working on the same problem. So I suggest that there should be an "I've got it" feature which allows a user who knows they can answer a question completely to label the question, so that other users don't exert themselves unnecessarily.
EDIT: The other users, of course, could still post if they felt they had something unique to contribute. But if the question felt straightforward, it would allow people to wait until an answer was posted so that they would not repeat someone's argument.
EDIT2: Today I posted one of $3$ comments within $6$ seconds of each other. But on a more serious note, I spent quite a bit of time solving a cubic equation from scratch, only to have someone else post a very similar solution a minute or so before I finished mine. It would be nice to at least know that someone else was working on the question...
EDIT3: Given the sensible points made by @Goos, I think the most effective version of this feature would
- work automatically, as opposed to requiring "opt-in", and
- work anonymously, saying "2 people are answering" as opposed to "x and y are answering." Despite this being the practice on Quora, I see little reason for specifying who is answering. That is invasive, and anyway what matters is that someone is answering.
Note also that there would technically be a sort of "opt-out", as @Asaf has mentioned :)
However, I would even be in favor of a much weaker version which could show "recent activity" on the question, for instance by given question views (as is already done) within the last few minutes. For example, if a simple question has 15 people looking at it, there is little reason to attempt writing an answer - this would gauge the same principle without being as invasive to one's privacy.