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The detail you might have missed in Prince William and Kate's birthday post for Archie

Prince Harry and Meghan's son is two

Ainhoa Barcelona

The Duke and Duchess of Cambridge celebrated their nephew Archie's second birthday on Thursday by sharing a very sweet throwback photo from his christening.

But did you notice one detail about the family snap that was posted on Twitter and Instagram? William and Kate chose to upload the same photo they did last year, when they wished their nephew a happy first birthday.

MORE: The Queen shares birthday tribute to her great-grandson Archie

Their captions were also strikingly similar, with this year's message reading, "Wishing Archie a very happy 2nd birthday today" alongside a balloon and a cake emoji. Last year's caption similarly read "Wishing Archie a very happy first birthday today!" and was accompanied by the same balloon emoji.

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WATCH: Happy 2nd birthday to Prince Harry and Meghan's son Archie!

The official portrait, which was taken by Chris Allerton in the Green Drawing Room of Windsor Castle, showed the Cambridges attending Archie's christening in July 2019.

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Prince Harry and Meghan, who was holding her baby boy, posed centre stage and were surrounded by Prince Charles and Camilla, the Duchess' mother Doria Ragland and Princess Diana's sisters Lady Jane Fellowes and Lady Sarah McCorquodale.

The photo William and Kate chose to post this year

It's perhaps no surprise that William and Kate chose to reshare this christening shot for their nephew's birthday given how beautiful a family photo it is. Harry and Meghan are also protective of their son and mindful of the pictures that are released to the public.

Prince Charles and Camilla similarly chose to reshare the same photo they posted last year for Archie's first birthday, which showed a touching moment between the future King and his fourth grandchild. The official black-and-white portrait, which also featured Prince Harry, was again taken at the christening.

They shared the same family photo last year for Archie's first birthday

While members of the royal family typically choose to share photos to celebrate birthdays, on Thursday the Sussexes instead marked their son's special day by asking well-wishers to pledge money to promote vaccine equity.

In a joint statement on their Archewell foundation website, Harry and Meghan said: "We will not be able to truly recover until everyone, everywhere, has equal access to the vaccine. And with that intention, we are inviting you to contribute whatever you can – if you have the means to do so – to bring vaccines to families in the world's most vulnerable places.

"For a donation of just 5 dollars, you can cover the cost of a dose for someone in need. And because we were able to secure matching support from a number of organizations, that $5 you give will automatically turn into 20 dollars – covering the cost of four doses."

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