"Prince Louis seems like baby Kate," one woman wrote.
"Michael Middleton is a very handsome man! Little Louis looks just like him! Beautiful pictures," another wrote.
"Is that Louis or his mama," another asked.
While little Prince George, six, is sometimes said to resemble his father Prince William, his sister, Princess Charlotte, five, has been said to resemble numerous other members of the family.
Little Prince Louis, two, has also been compared to Kate's brother James Middleton.
It comes after the family shared another royal snap to celebrate Prince William's birthday on Sunday.
The family were believed to have been photographed in the second photo at their home in Anmer Hall where they've been self-isolating since the beginning of the coronavirus pandemic.