BUG: DataFrame.to_dict when orient=index data loss

doc/source/whatsnew/v0.24.0.txt

@@ -804,6 +804,7 @@ Reshaping
 - Bug in :meth:`Series.replace` and meth:`DataFrame.replace` when dict is used as the ``to_replace`` value and one key in the dict is is another key's value, the results were inconsistent between using integer key and using string key (:issue:`20656`)
 - Bug in :meth:`DataFrame.drop_duplicates` for empty ``DataFrame`` which incorrectly raises an error (:issue:`20516`)
 - Bug in :func:`pandas.wide_to_long` when a string is passed to the stubnames argument and a column name is a substring of that stubname (:issue:`22468`)
+- Bug in :func:`DataFrame.to_dict` raises ``ValueError`` when used with ``orient='index'`` and a non-unique index instead of losing data (:issue:`22801`)
 
 Build Changes
 ^^^^^^^^^^^^^

pandas/core/frame.py

@@ -1163,6 +1163,10 @@ def to_dict(self, orient='dict', into=dict):
                            for k, v in zip(self.columns, np.atleast_1d(row)))
                     for row in self.values]
         elif orient.lower().startswith('i'):
+            if not self.index.is_unique:
+                raise ValueError(
+                    "DataFrame index must be unique for orient='index'."
+                )
             return into_c((t[0], dict(zip(self.columns, t[1:])))
                           for t in self.itertuples())
         else:

pandas/tests/frame/test_convert_to.py

@@ -71,6 +71,12 @@ def test_to_dict_timestamp(self):
         tm.assert_dict_equal(test_data_mixed.to_dict(orient='split'),
                              expected_split_mixed)
 
+    def test_to_dict_index_not_unique_with_index_orient(self):
+        # GH22801
+        # Data loss when indexes are not unique. Raise ValueError.
+        df = DataFrame({'a': [1, 2], 'b': [0.5, 0.75]}, index=['A', 'A'])
+        pytest.raises(ValueError, df.to_dict, orient='index')
+
     def test_to_dict_invalid_orient(self):
         df = DataFrame({'A': [0, 1]})
         pytest.raises(ValueError, df.to_dict, orient='xinvalid')