If your student failed a lesson, they will be prompted to attempt the lesson again. Please see details in the article What happens if my student fails a lesson?.
If you believe your student is repeating the same lesson multiple times, it's possible that they are completing lessons with similar items. A good way to check to see if they are repeating lessons is to view the Student Progress Report.
If you still feel there is an issue, please contact Technical Support at email@example.com and include the student's username, lesson in question, and any additional information you may have. We'll be happy to help you troubleshoot!