I think it could be the "note to Frank." You actually have to collect it twice. After Bill reads it, he throws it away on the ground. You have to pick it up again and it counts as an additional artifact.
That would probably be it. I didn't realise this could happen.
After Bill reads the note, he screws the piece of paper into a ball and tosses it on the floor. The second time you pick up the note, it looks very different to the first time you collected it, making it a unique artifact.