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
```
Where? Were you looking at an older version from yesterday where I had a pluck?

I refactored and turns out ".uniq" is needed to ensure unique results among combined scopes (not even sure why that's not the default).