@hhope - With your MS Forms Quiz open, select the breadcrumbs at the top right of the Forms screen. Select Settings. In the section called Who Can Fill Out This Form, deselect the One Response Per Person checkbox. Then finish creating your quiz. Unfortunately, you cannot universally set all of the quizzes you create to this option (hopefully MS will add this feature in the future). Also, from my experience, you cannot change this setting once you have sent out your form. Either you do it at the beginning or better luck with the next quiz.
