I do not find pictures of Sarah Ferguson wearing this pearl choker previously.
This Amfar gala took place during the Cannes film festival on May 21, 2015. During this festival, a lot of actresses, models, etc., wear jewels loaned by jewellers, so it is posible that Sarah Ferguson borrowed this pearl choker. But I would be very happy if other posters can find pictures of Sarah wearing this choker previously!
Note that the bracelet worn by Sarah Ferguson is interesting too!
Sarah, Duchess of York attends amfAR's 22nd Cinema Against AIDS Gala, Presented By Bold Films And Harry Winston at Hotel du Cap-Eden-Roc on May 21, 2015 in Cap d'Antibes, France. (Credit: Tristan Fewings/Getty Images Europe)