In my experience (again), we do have two rounds to separate them. But both of them are voting-based rounds. One vote to narrow down the 30+ ideas to like 5 or so, and a second one to reduce it to one or two :) The outcome was very minimal compared to the overall time wasted on the process (5-6 people in a room, for one hour, ending up with a not-so-interesting idea... there are better ways to spend a team's time).