Duplicate namespec in Person.namespecs()

When listing a Person's namespecs, if there is a volume with frontmatter, 2 namespecs have the volume as their `parent` while none have the frontmatter Paper.

This happens for both old-style and new-style IDs:

```py
>>> p = anthology.get_person('shay-b-cohen')
>>> [ns for ns in p.namespecs() if ns.parent.full_id=='W15-15']
[NameSpecification(Name('Shay', 'Cohen'), id='shay-b-cohen'), NameSpecification(Name('Shay', 'Cohen'), id='shay-b-cohen')]
>>> [ns for ns in p.namespecs() if ns.parent.full_id=='W15-1500']
[]
>>> [ns for ns in p.namespecs() if ns.parent.full_id=='2024.scalellm-1']
[NameSpecification(Name('Shay', 'Cohen'), id='shay-b-cohen'), NameSpecification(Name('Shay', 'Cohen'), id='shay-b-cohen')]
>>> [ns for ns in p.namespecs() if ns.parent.full_id=='2024.scalellm-1.0']
[]
```

In fact these are identical namespecs:

```py
>>> a, b = [ns for ns in p.namespecs() if ns.parent.full_id=='W15-15']
>>> a is b
True
```

From a user perspective, this is confusing—why list the same namespec twice?