She's worn it at least once before. I think HM gave away pieces of her jewelry collection before she died and this necklace is one of the things Anne received.

https://finance.yahoo.com/news/princess-anne-wears-mother-queen-161843410.html?guccounter=1&guce_referrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8&guce_referrer_sig=AQAAAGjzAlWYlkOXSONJSCK4bQz4Qi-WEvCVB38TOXZ5DhyfX55ovQOWhSs7klPOhdzURRgmCGW3asl7lxvEyryYMdhesnpjwnGbV4uAB_yHEGDqr1-HbUp0Z_1dLWC1WL2TiG6z8_ZOirAqdT9Rpe2YsAHf0XnSzCUmrBpRzcxmEIYe