If you leave the duty while others are still viewing CS and the circle barrier is still up, you don't incur the 30 minute penalty for leaving.

Additionally, what is happening is that people are doing Low Level Duty Roulette for the Myth or XP bonus and hoping to get either Ifrit, Garuda, or Titan story mode. Where myth is concerned: 5 minutes for 60 Myth > 20-30 minutes for 60 Myth from running a low level dungeon.

This problem will continue to exist until SE creates a "Low Level Trials" for roulette and separates it from the dungeons.