Exactly, don't want to do a certain dungeon/trial? Don't queue through Duty Roulette! Roulettes assign you to a duty based on how many lower level players are specifically queuing for that trial (and 'Main Scenario' ones are only two so there is a 50/50 chance you'll get one or the either anyway) - it's not simply an easy way of getting tomes, that's just a carrot to get high level players doing those duties to help new players out. So if you're getting the same duty over and over then maybe new players need it and it's working as intended.

Don't think the roulette carrot is worth the effort of doing that duty repeatedly? Don't do roulettes then! It's as simple as that.