Tessy of Nassau's black & white evening gown worn for the National Day's gala is also an old one worn by the Grand Duchess about ten years ago (By Lacroix if I remember correctly).
It was altered by taking it in and lengthened by adding extra fabric (a little too obviously) at the hemline.
While it might be called 'generous' of the GD to let her daughter-in-law wear her clothes, it's also a bit weird. Wearing her old dresses as a 'homage' would be nice of Tessy, but practically dressing the poor girl overall in hand-me downs which she could not very well refuse to wear strikes me as a rather embarrassing situation.