1) Set up a course if required.
2) Add a single user to two roles on that course (e.g. Student, Non-Editing Teacher)
3) Create any activity on that course e.g. forum.
4) Edit the forum settings and click to the roles tab. Click on a role type to assign, e.g. Student again.
The user who was added to the course should appear once in the list of potential candidates to be added to the role.
The user appears twice.