To reproduce:

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.

Expected result:

The user who was added to the course should appear once in the list of potential candidates to be added to the role.

Actual result:

The user appears twice.