[adityaanikam]

Fixes #989

Since 4.2.6, _concat_arrow_tables always calls pyarrow.concat_tables(..., promote_options="default"). Arrow's schema unification rejects duplicate field names, so any result with two columns of the same name (for example SELECT 1 AS a, 2 AS a) fails on fetch with ArrowInvalid: Can't unify schema with duplicate field names, even when every chunk has the same schema.

The chunks are now concatenated directly when all their schemas are equal. The promote path is only used when the schemas differ, so the nullability case from #418 still works. A result that has duplicate names and differing chunk schemas still fails, as before.

Tests (tests/unit/test_util.py): added a duplicate-column-names test and a nullability-promotion test. I also changed the old-pyarrow fallback test so its second chunk differs in nullability. Otherwise the two chunks would now skip the promote path and the fallback would no longer be exercised. The duplicate-names test fails without the change. pytest tests/unit -m "not realkernel" passes (1264 passed), and black --check is clean. I only ran this on Windows with Python 3.12 and pyarrow 26, not against a real warehouse or with older pyarrow versions.

Added a line to the Unreleased section of CHANGELOG.md.

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