פירוש על קידושין 7:7

This very derashah undermines what the Talmud perceives to be a rule of exegesis—no word may be interpreted twice. The word “eyn” was used to derive that both grandchildren and illegitimate offspring disqualify her from eating terumah. To resolve the difficulty, the Talmud says that the first derashah, concerning grandchildren, was not needed. Grandchildren are treated like children (usually treated much better than children, I might add). The derashah was needed to teach that illegitimate children also prevent her from eating terumah.