This is usually the result of mistyping a student's name in the subject
in question.  To avoid this problem you should use the copy student option
when setting up different subjects instead of trying to re-type the student
list for each subject.  To correct this situation once it has already happened
you should go to the subject in question and re-type the student's name
carefully makeing sure it is exactly the same as it appears in the other
other subject's student lists.  Common mistyping errors include different
spacing, a trailing space, or differing capitalization.

ie.

"Aldridge, Shawn " vs "Aldridge, Shawn"
"Clark, Joe" vs "Clark, joe"
"Smith,John" vs "Smith, John"
" Doe, Jane" vs "Doe, Jane"