I assume since littlefinger revealed the necklace was made only a week or two before the death, they had more than one in order to use in the trial. It wasn't a truly unique one like the fool spun it to be.
About 1/4 through book 2 now. Confused, though. They are talking about Ramsay Snow having Reek, but in the book, Theon has only just arrived at his Father's and hasn't done any of the stuff that got him placed in Ramsay's grasp yet.