parse_held_out_specs silently drops a duplicate held-out column name instead of erroring

[yakew7]

Where: faircode/proxy.py:34-62 (parse_held_out_specs).

The gap: the function already raises ValueError if a held-out column collides with a real column already in the profiled dataset (line 53-56), but never checks collision between two held-out specs themselves - held_out[column] = ... just overwrites silently.

Repro:

>>> import pandas as pd
>>> from faircode.proxy import parse_held_out_specs
>>> df = pd.DataFrame({'sex': ['M', 'F'] * 5})
>>> def read_table(p):
...     return pd.DataFrame({'race': ['A', 'B'] * 5}) if p == 'a.csv' else pd.DataFrame({'race': ['X', 'Y'] * 5})
>>> held = parse_held_out_specs(['a.csv=race', 'b.csv=race'], df, read_table)
>>> held['race'].tolist()
['X', 'Y', 'X', 'Y', 'X', 'Y', 'X', 'Y', 'X', 'Y']

a.csv=race is silently discarded - no error, no warning - only b.csv=race's data survives.

Why it matters: --proxy-hints-with/held_out_with is designed to accept multiple specs (testing several dropped protected attributes at once); an accidental repeated column name - a typo, a copy-paste, or two different candidate source files meant for the same attribute - silently loses one of the two tests instead of erroring, the same way the sibling real-column collision check already does. This repo's own stated design philosophy elsewhere is loud failure over silent data loss.

Suggested fix: raise ValueError in parse_held_out_specs when column in held_out before assigning, mirroring the existing real-column collision check's error style.

[yakew7]

Fixed in 39fc54f — parse_held_out_specs now raises ValueError on a duplicate held-out column name, mirroring the existing real-column collision check.