To redo a question: When a student has completed a task, they can look at their scorecard and choose any of the questions to try again. To do this, they just click the 'Practice this problem again' button.
This does not affect the grade, as we think it's important for teachers to see the misconceptions students might have had. But the students do get extra 'credit' by getting the points allocated to that question.
Redoing a whole task: For a student to do the whole task again, the teacher needs to duplicate the task and reassign it to just the one student or group of students that want to redo it.