Ok, here's the deal: There are two Collars of Khorne. One of them in core rulebook (pg301), it's a Khorne Reward that grants auto-passing Blood God's Contempt. The other one is in The Tome of Blood (pg44), it's a purchasable Extremely Rare item that counts as a Null Rod (and prevents Force Weapons activation).
What should I do about it? Treat them as two separate items (rename one for clarity)? Remove one of them and only use the rules of the other? If accepting both of those, should they be stackable?