In a double-page spread in Wednesday's paper, Ms Platell pleaded "for pity's sake, leave her alone" in a piece that complained of constant press articles attacking the Duchess's appearance, saying she is going grey, her eyeliner is too thick and she is too thin for a new mother.
She may have forgotten her words appear in The Daily Mail, which has, together with its sister site MailOnline, published seven pieces about the Duchess of Cambridge in the last week focussing on her appearance, a quick search of the site's archive reveals.
In July, Mail columnist Sebastian Shakespeare wrote that her greying hair was "a disaster!"
The column quoted hairdresser Nicky Clarke saying she must dye any grey, saying: "‘It’s different for men. Men can go grey in their mid-50s and still be considered attractive — it’s the whole silver fox thing — but it’s not the same for women."
Ms Platell's piece does not mention her own history of criticising the Duchess, such as in May, 2012 when she wrote: "Forgive me your Royal Hairness but it's time for the chop!"
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She wrote that the Duchess had "looked like Cousin It from the Addams Family — all hair, no person" at one public engagement, lamenting: "It was so distracting."
In May last year, Ms Platell was "running out of sympathy" with the woman she now defends, after she suffered a "wardrobe malfunction".
Ms Platell called the photo of her exposed behind "humiliating" and said: "Kate appears to be wearing a thong that would make even a cheese cutter wince.
"How mortifying for any woman to be pictured in such an unedifying way," Ms Platell wrote. "I couldn’t help but think that surely Kate could have tried a little harder to keep her modesty under wraps? As a relatively seasoned member of the Royal Family, isn’t it time she made more of an effort to cover up?"
One person described the Mail columnist as "the personification of the Daily Mail's lack of self-awareness".
The irony of this article being published in the most judgemental paper of all cannot be lost - https://t.co/cjosLAi1S8— Karenza Morton (@renzam12) October 28, 2015
Ms Platell's column today did not reference her past criticisms of the Duchess. It does provide a word of warning of how Kate should not behave.
She referred to the fact that Kate's husband's deceased mother Princess Diana did far more public engagements than Kate. But Ms Platell offered a sympathetic warning not to do the same.
"But then again, look where that got Princess Diana. Burnt out, bulimic, emotionally unstable, chasing unsuitable suitors and craving the limelight. It was a recipe for tragedy."
You've been warned, Kate.