I am sorry, but the Duchess of Windsor's ruby and diamond necklace by Van Cleef & Apels is definitely NOT the same as the one worn by Princess Diana.
Both necklaces have a ruby tassel placed asymmetrically in the front, so there is some resemblance at first sight, but in detail there is not. For example, the Duchess of Windsor's necklace had a ruby thread entwined with the diamonds around the neck, while Princess Diana's is all diamonds around the neck.
Diana never wore a piece of jewelry formerly belonging to the Duchess; the only member of the British royal family who did was Princess Michael of Kent.
See also the respective topics about the Duchess of Windsor's jewels, Princess Diana's jewels etc. -