1) Two users have the same email (odd situation, but possible).
2) One of them (John) is enrolled in "Course A" inside group "Class 3", but the other (Susan) is not enroled in any course.
3) In course A there is a group-separated assignment, where John has uploaded his work.
4) A teacher tries to grade this assignment, choosing from the drop down menu the "Class 3" group.
5) Both John and Susan appear as gradable users, even when Susan is not enrolled in Course A.
I'll try to dig into the code as soon as I can.