Prince William and Kate Middleton
Kate Middleton and Prince William both rewear their outfits in their joint engagement at Mental Health Summit. Pictured: Prince William and Middleton sit on a 'friendship bench', as they attend the first Global Ministerial Mental Health Summit, co-hosted by the UK Government and the OECD, in London on Oct. 9, 2018. Getty Images/Eddie Mulholland

Kate Middleton and Prince William just had their first joint engagement following Prince Louis' birth.

On Tuesday, the Duke and Duchess of Cambridge stepped out together to attend the first Global Ministerial Mental Health Summit in London. The event aims to improve mental health around the world. According to Sabrina Carder, a journalist for Evening Standard, both royals opt to recycle their wardrobe.

For the said engagement, the royal mom-of-three opted to rewear a purple A-line dress by British designer Emilia Wickstead. It features a flattering waist belt. She also paired it with the same heels. Middleton first wore it in 2017 in Hamburg during their official visit in Germany with Princess Charlotte and Prince George.

The duchess has been popular for recycling outfits. She re-style her wardrobe with a new taupe handbag with faux croc detail by another British brand, Aspinal of London.

Middleton was not the only one who recycled an outfit for the event because her royal husband did it too. Prince William wore the same suit and tie combination he wore in Hamburg, too.

In related news, the said outing was Middleton's second royal engagement following her six-month maternity leave. She made her official return by visiting St. Augustine's CE Primary School in Kilburn, London, last week. During the said outing, the Duchess of Cambridge broke several royal protocols.

Middleton was photographed pushing back the car door after stepping out. Prior to that, her sister-in-law Meghan Markle made headlines for shutting down her car door. According to many, royals should not do it for security reasons. However, it turned out that Middleton has been doing it in the past. In fact, there was a video where the Duchess of Cambridge closed her car door while carrying Princess Charlotte.

In addition, Middleton wore pants. She is rarely seen in trousers compared to the Duchess of Sussex. However, Middleton has been photographed in pants multiple times in various engagements in the past. Also, Middleton was photographed hugging the children.

Middleton and Prince William's next joint engagement has not been announced yet. However, they are both comfortable attending duties on their own, especially when it comes to subjects close to their heart, according to OK! royal expert Christina Reeves. For instance, Middleton didn't join Prince William in his recent trip to Africa for the Wildlife and Tusk Trust.