I also noticed this same issue. Duplicate Editor and Viewer permissions defined and unable to delete or add to permissions with the same error shown in the image of post #1.
After investigation of the database I noticed that there were duplicate rows for Editor and Viewer in the dashboard_acl table. I am using enterprise Grafana v7.5.3 on CentOS and the sqlite3 database that is there by default.
There are unique constraints defined on the dashboard_acl table but it appears that when one of the column values is NULL that this constraint is ineffective. The duplicate rows had a dashboard_id of -1 and NULL values for user_id and team_id.
In summary the way I resolved this issue was to delete the extra duplicate rows from the sqlite3 database used by my Grafana install. I identified the rows to delete based on created/updated column values and compared rows to a clean install. Just make sure you take a back-up or are able to insert anything you delete. I don’t think there were any cascading deletes but I did not check as thoroughly as I should.
Once the rows were deleted I was able to add/remove permissions in the GUI without error and the duplicates were no longer present.
No idea why this was affecting my Grafana install. It is something I inherited and already came with the issue. Never seen it on any clean new installations.