Same group can be entered twice in branch permissions

Summary

The same group can be added multiple times to the same branch permission.

Steps to Reproduce

1. add branch permissions for a group over REST, using different casing than that used for the group initially
2. in the UI, edit the existing permission and type in the name of the group again

Expected Result

The group will not be listed and cannot be added.

Actual Result

The group is listed and can be added to the permission