So, maybe I'm misremembering how extreme the double-roll is for fest bosses. But I could've sworn that a double-roll is ~6 items per boss. As seen below, however, my level 75 team has only been getting 3 (or 4 if I get an av. item like in this one) items per boss encounter. I'm not sure if other people are getting this, or if this is even a glitch or if I'm just misremembering the amount, but I'm 99% sure this isn't a double-roll and I've been getting this amount of rewards on every boss.

For Avatar Items, you get two rolls. But it is not guaranteed to get a drop. However, sometimes you can get 3 or 4 items to drop from one boss.

For non Avatar Items, it uses a different loot table where the rewards are increased and where you have higher rates of the rarer drops, like getting Catalysts.