http://eu.battle.net/wow/en/game/patch-notes/4-2-0
Set Bonuses
The 2-piece resilience bonus from Cataclysm PvP sets no longer stacks. If a player wears 2 pieces each from 2 different Cataclysm sets, the player will only receive the 400 resilience rating bonus once. This change does not apply to PvP sets from prior expansions.
