fixes #8775 - ensure available errata are unique

The same errata may be in multiple repositories, resulting in duplicate errata:

```
system.available_errata.map(&:errata_id).count = 42
system.available_errata.map(&:errata_id).sort.uniq.count = 26
```
We typically do "#{Erratum.table_name}.id" for easier refactoring