Blizzard have detailed a hotfix coming later this week as well as several improvements coming to the Group Loot system in patch 10.0.5
Group Loot (Source)
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.
This has already been working as intended for most items, but should now include items such as Tier Tokens and weapons.
Additionally, we’ve got some updates to loot distribution coming in 10.0.5–
Fixed an issue where, in some rare cases, players would get less loot than they should have on certain Raid sizes.
Fixed an issue where players could unconditionally roll Need on Set Tokens. The system will now check if the player owns the Token or the Set Piece itself at equal or greater power.
Players can no longer roll Need on any Profession recipe that they have already learned.
Players can no longer unconditionally roll Need on 1H weapons (or 2H for a Warrior in Fury loot spec).
For that last one, in 10.0.5, the system will now thoroughly identify if you can dual-wield the weapon in question, based on your loot spec. If you can, and you already have two versions of it at least as powerful as the item being rolled for, you can’t Need the item.