BUG: make JSONTableWriter fail if no index.name and 'index' in columns

doc/source/whatsnew/v3.0.0.rst

@@ -559,6 +559,7 @@ MultiIndex
 I/O
 ^^^
 - Bug in :class:`DataFrame` and :class:`Series` ``repr`` of :py:class:`collections.abc.Mapping`` elements. (:issue:`57915`)
+- Bug in :meth:`.DataFrame.to_json` when ``"index"`` was a value in the :attr:`DataFrame.column` and :attr:`Index.name` was ``None``. Now, this will fail with a ``ValueError`` (:issue:`58925`)
 - Bug in :meth:`DataFrame.to_dict` raises unnecessary ``UserWarning`` when columns are not unique and ``orient='tight'``. (:issue:`58281`)
 - Bug in :meth:`DataFrame.to_excel` when writing empty :class:`DataFrame` with :class:`MultiIndex` on both axes (:issue:`57696`)
 - Bug in :meth:`DataFrame.to_stata` when writing :class:`DataFrame` and ``byteorder=`big```. (:issue:`58969`)

pandas/core/generic.py

@@ -2387,7 +2387,8 @@ def to_json(
         index : bool or None, default None
             The index is only used when 'orient' is 'split', 'index', 'column',
             or 'table'. Of these, 'index' and 'column' do not support
-            `index=False`.
+            `index=False`. The string 'index' as a column name with empty :class:`Index`
+            or if it is 'index' will raise a ``ValueError``.
 
         indent : int, optional
            Length of whitespace used to indent each record.

pandas/io/json/_json.py

@@ -59,6 +59,7 @@
 from pandas.io.json._table_schema import (
     build_table_schema,
     parse_table_schema,
+    set_default_names,
 )
 from pandas.io.parsers.readers import validate_integer
 
@@ -353,6 +354,8 @@ def __init__(
             raise ValueError(msg)
 
         self.schema = build_table_schema(obj, index=self.index)
+        if self.index:
+            obj = set_default_names(obj)
 
         # NotImplemented on a column MultiIndex
         if obj.ndim == 2 and isinstance(obj.columns, MultiIndex):

pandas/tests/io/json/test_pandas.py

@@ -1610,6 +1610,13 @@ def test_to_json_from_json_columns_dtypes(self, orient):
         )
         tm.assert_frame_equal(result, expected)
 
+    def test_to_json_with_index_as_a_column_name(self):
+        df = DataFrame(data={"index": [1, 2], "a": [2, 3]})
+        with pytest.raises(
+            ValueError, match="Overlapping names between the index and columns"
+        ):
+            df.to_json(orient="table")
+
     @pytest.mark.parametrize("dtype", [True, {"b": int, "c": int}])
     def test_read_json_table_dtype_raises(self, dtype):
         # GH21345