Want to know why? The cream Alexander McQueen number is actually the dress that Kate wore to Prince George's christening all the way back in October 2013.
Arriving at the event with her husband Prince William, Kate greeted the 'unsung heroes' who had helped to deliver both of her children - a 23-strong team from the St Mary's Hospital, Paddington, in 2013 and 2015, whom the couple had invited to the prestigious occasion.
The Duchess of Cambridge pictured in the same designer dress at George's christening
Consultant anaesthetist Dr Johanna Bray, a member of staff from the hospital, revealed, 'We weren't actually at the birth, but were behind the scenes. We were all on call for three months - my husband basically did the childcare for three months.'
'You never know when you need to be called, you need to be in town, and available. If you are at a party you need to have your car keys at the ready. No drinking!'
Professor Tiong Ghee Teoh, a consultant obstetrician and gynaecologist, added that Kate has been 'very appreciative' of the care provisions set out for her, and was surprised that 'there were a lot more people behind the scenes than she realised'.
'She was a fit young woman, that's the most important thing. At the end of the day you want to deliver a healthy, happy baby to a a healthy happy mother - and that’s what happened,' he said.
Kate, William and Harry were all smiles as they were guided around the flower show in Chelsea
It's been a busy week of outdoor events for the royals - the day before, William and Kate attended the Chelsea Flower Show with Prince Harry, where they had the chance to see a pink and green chrysanthemum that has been named in honour of their daughter Charlotte.
As they walked through the event, the Duchess also let slip her personal nickname for her husband. William was overheard asking award-winning Australian gardener Charie Albone which plant was his prize Buxus shrub, to which Kate responded, 'Babe, we've got those. We've got loads of those.'
'Yeah she actually called him "babe"', he confirmed in an interview with Channel 9's Today Show.