Blizzard has posted upcoming changes to Group Loot in raids which includes changes to Need rolls for Tier Tokens. Need rolls for tier will have lower priority if you already own that set piece at an equal or higher item level in Patch 10.0.5.
We have a hotfix coming to the game later this week. With this hotfix, we will have:
Fixed some situations where players could win Need rolls on duplicate items in a single roll session when it should not have been possible.
