Fix _col=<pk> producing duplicate column in output

[riteshkew]

Fixes:

• _col=id can cause id column to export twice in CSV export #1975

When _col=<pk_column> is passed (e.g. ?_col=id&_col=title&_col=body on a table whose pk is id), _columns_to_select starts columns with the pks and then extends it with the deduplicated _col list — but the dedup is only within _col itself, not against the pks already in columns. So id ends up in the SELECT twice:

$ /simonwillisonblog/blog_entry.csv?_col=id&_col=title&_col=body&_size=1
id,id,title,body
1,1,WaSP Phase II,...

This skips _col values that already appear in columns (i.e. the pks) when extending:

columns.extend(c for c in dict.fromkeys(_cols) if c not in columns)

Behavior notes:

• Pks still appear first in the configured order; ordering of additional _col values is unchanged.
• Existing _col=state&_col=state dedup-within-_col behavior is preserved.
• _nocol interaction is unchanged — it still filters the final list.

Added two regression cases to test_col_nocol, both linked to the issue:

• ?_col=pk&_col=state → ["pk", "state"]
• ?_col=pk → ["pk"]

Also probed multi-pk tables (pk=[a,b]) and pk-only requests; both produce the expected dedup behavior.

Prepared with AI assistance.

[simonw]

I confirmed that the new tests fail without this fix:

    async def test_col_nocol(ds_client, path, expected_columns):
        response = await ds_client.get(path + "&_extra=columns")
        assert response.status_code == 200
        columns = response.json()["columns"]
>       assert columns == expected_columns
E       AssertionError: assert ['pk', 'pk'] == ['pk']
E         
E         Left contains one more item: 'pk'
E         Use -v to get more diff

tests/test_table_api.py:1464: AssertionError
================================================================== short test summary info ==================================================================
FAILED tests/test_table_api.py::test_col_nocol[/fixtures/facetable.json?_col=pk&_col=state-expected_columns4] - AssertionError: assert ['pk', 'pk', 'state'] == ['pk', 'state']
FAILED tests/test_table_api.py::test_col_nocol[/fixtures/facetable.json?_col=pk-expected_columns5] - AssertionError: assert ['pk', 'pk'] == ['pk']

[simonw]

Thanks for this!