Uploading graders through CSV allows for a grader to be assigned to a group multiple times

When uploading grader-group mappings (via Graders -> Upload) _without_ the `Remove existing grader assignments` option selected, a grader can be assigned to the same group multiple times.

## Steps to recreate
1. Create a CSV mapping a group to a grader.
2. Upload the grader assignment.
3. Upload the grader assignment _again_ (and do _not_ select `Remove existing grader assignments`)