`NestedFrame.__setitem__("new_nested.x", flat_values)` repeats values for non-unique index

**Bug report**

```python
from nested_pandas.datasets import generate_data

nf = generate_data(5, 3)
nf['new.flux'] = nf['nested.flux']
print(nf)
```

```
a ... new
0 0.768677 ... [{flux: -85.021725}; …] (6 rows)
0 0.618566 ... [{flux: -85.021725}; …] (6 rows)
1 0.308658 ... [{flux: -16.160369}; …] (9 rows)
1 0.612433 ... [{flux: -16.160369}; …] (9 rows)
1 0.332888 ... [{flux: -16.160369}; …] (9 rows)
```

Note 6 and 9 nested rows instead of 3

**Before submitting**
Please check the following:

- [x] I have described the situation in which the bug arose, including what code was executed, information about my environment, and any applicable data others will need to reproduce the problem.
- [x] I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a description of what I expected instead.
- [ ] If I have a solution in mind, I have provided an explanation and/or pseudocode and/or task list.

Start by running the reproduction using generate_data and the NestedFrame.__setitem__("new.flux", flat_values) assignment. Trace how the non-unique outer index is handled, then verify that each outer row receives the expected three nested rows rather than repeated values.

Written by the indexing model from the issue text.