If using 7.82 of NetClassroom: This issue can be caused by students clicking Refresh on the Enter course requests page. There is a message indicating that the Back, Forward, and Refresh buttons should not be used when entering requests, and it might be beneficial to add an additional reminder in the form of an Announcement or directions that can be added when setting up registration in Configuration.
This issue has also been reported with students who are using the Safari browser. Safari is not supported for use with NetClassroom.
NOTE: Safari is supported in version 7.83 of NetClassroom and higher.