You could write a plugin that gets activated when the user is in answer mode (clicked the answer button).
Then you ajax request each 20 seconds (or so) if the number of answers has changed in the database. If so, then you can inform the user frontend about an incoming answer.
Technically not difficult.
However, think of:
1. It is better to have more answers. They are mostly very different, I would not confuse the user by saying, "hey, there is an answer just now. Stop your 5 min text and trash it."
2. Server load.