Tested with fear, daze, and stun. The most recently applied CC will overwrite the other and remove the previous CC, regardless of duration. A 0.5 second daze will remove a 3 second stun, a fear will remove a daze, and so on. Tested in the Mists.
This is borderline gamebreaking for PvP, especially for a class that relies on the interactions they have with stuns like a warrior.
Did not test with basilisk venom, knockbacks, or knockdowns. Immobilize was tested and it works fine.
EDIT: Sorry, I made this in the wrong forum. I am going to repost it in the appropriate forum. Please lock.