Catherine wore L.K. Bennett Madison dress in July 2019 when she took George, Charlotte and Louis to a polo match where Prince William and Prince Harry were playing.
The dress was described as “Cut from soft and sumptuous silk, this flowing dress cascades over the figure to a demure midi length and features loose-fitting sleeves and a beautiful all-over print.”
A closer look at the small animal print. It is an old style of the label so I believe Duchess has it in her wardrobe for a long time as we have seen her wearing the same dress in purple in August 2015.