Duncan had indeed cast a Fire Seed into that mirror image of the Frost City-State—but he felt it was far from enough.
He could distinctly feel that the situation with Frost was different from that with Prand; the Fire Seed he cast did not spread swiftly on "the other side of the mirror," and his perception of the Fire Seed was greatly weakened and interfered with—he guessed that this might be because there were natural barriers between the worlds "on both sides of the mirror," or because there was not an exact correspondence between the mirror image and the real world, and those misaligned parts were interfering with his judgment.
Whatever the cause, he needed to find a way to strengthen the connection between himself and the Fire Seed, as well as the connection with the White Oak and Agatha.