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The young royal hosted an event in aid of In Kind Direct, a charity supported by the heir to the British throne, at London's Clarence House.
St James's Palace said the duchess had been "delighted to be able to step in" and greet the 30 guests attending the reception.
Charles had traveled to Saudi Arabia for the state funeral of crown prince Sultan bin Abdul-Aziz Al Saud.
William and the duchess plan to travel to Denmark next week to visit a UN centre that distributes emergency supplies to East Africa.