Are artifacts intended to be unique?

[Yamstis]

I was under the impression that yellows were removed from the pool when they're generated, but I've just had Tooth of the Mouth pop up twice in one game. Once received from the lumberjacks, and once from a boss. I'm just wondering if I've misunderstood something about how these items work, or if this is a known quantity that the lumberjack reward doesn't remove it from the pool.

[ghostbuster]

Artifacts are supposed to be unique.
But there is a recurring bug that duplicates some of them from time to time.

[HousePet]

There are two places that ignore the uniqueness stat.

[Radon26]

as far as it is know there are 2 at least.