@@ -934,6 +934,13 @@ def _concat_arrow_tables(table_chunks: List["pyarrow.Table"]) -> "pyarrow.Table"
     Promotion is only needed when chunk schemas actually differ (e.g. the
     nullability case from GH #418), so when every chunk shares the first
     chunk's schema we concatenate without promotion.
+
+    Known limitation: a result set that has *both* duplicate column names *and*
+    chunks whose schemas genuinely differ falls through to the promotion path
+    and still re-raises ``ArrowInvalid: Can't unify schema with duplicate field
+    names``. Arrow cannot unify a schema with duplicate field names, so there is
+    no promotion-based fix for that combination without renaming fields; it is a
+    narrow, pre-existing edge left unhandled here.
     """
     if all(table.schema.equals(table_chunks[0].schema) for table in table_chunks[1:]):
         return pyarrow.concat_tables(table_chunks)