The neckline on her recent outfit is very similar to Meghan's style.
The young royals have been strong advocates for mental health, setting up the Heads Together campaign in 2016 through their official charity, The Royal Foundation.
Kate's appearance came ahead of World Mental Health Day on Wednesday.
Kate and William also met with Dairo Vargas, a contemporary fine artist from Columbia, who is creating an original piece of art at the summit, inspired by the day's events.
Prince William joked that the painting had been ruined.
HELLO! magazine reports that Kate was shown a pencil drawing by William Henry Fox Talbot, who has been referred to the "father of photography".
This saw the couple break out into laughter along with the crowd.
Interpol president Meng Hongwei reported missing after trip to China
Neither Interpol nor Chinese authorities have made any official statement about Meng's disappearance or the investigation so far. Meng Hongwei, the Chinese head of the worldwide police organisation Interpol , was reported missing by his wife in September.
However, fans were quick to notice that Middleton, being the queen of recycling, had worn the dress somewhere before: during her family's visit to Germany back in July 2017.
The project's founder Dixon Chibanda said: "Bringing our Friendship Bench to London helps show leaders and campaigners how we can all empower communities to tackle the global gap between mental health problems and the treatments available".
This was Catherine's second engagement following her return to work, having taken taking maternity leave after the birth of the couple's third child, Prince Louis, in April.
The duchess first showed off the Emilia Wickstead dress on a trip to Germany previous year.
The Cambridges have two older children - Prince George, five, and Princess Charlotte, three.
Erdem has become somewhat of a go-to designer for members of the royal family, with the label being the favourite to design Princess Eugenie's wedding dress.