While they don't state a minimum, you should aim for a very high score, generally considered to be 100 or above to be competitive. Scores in the 110+ range are even better. Remember that a high TOEFL score is just one component of your application. Strong academics, compelling essays, letters of recommendation, and extracurricular activities all play crucial roles.
It's best to check Duke's official website for the most up-to-date information and to review their application requirements carefully. Contacting their admissions office directly may also provide further clarity.