This can only happen if a student was accidentally given the secret access code from another student - for example, two students with the same name but different student numbers.
The student can fix this by logging into simplegradebook and selecting "Add or Edit Classes" from the Classes menu. The student will see something like the following display:
(https://smf.newcastle-technology.com/index.php?action=dlattach;topic=166.0;attach=99;image)
At the bottom, the student can delete the access code that does not apply. Because the student numbers are part of the access code it should be easy to identify the correct one to delete.