Kate, 43, typically claps with her hands flat rather than clasped around one another.
At the event, George was seen clapping in the same way – something eagle-eyed fans were quick to point out.
One fan wrote on X: “They clap the same.”
Another said: “Prince George might look like a Spencer, but he claps just like his mum.”
Other royals to attend the event last night included King Charles and Queen Camilla.
The Duke and Duchess of Edinburgh, and The Duke and Duchess of Gloucester, were also at the Remembrance concert.
Prince William did not attend as he landed back in the UK from Brazil just hours before.
It is believed he stayed at the family’s new Forest Lodge home with Princess Charlotte, 10, and Prince Louis, seven.