Fix index duplication

xarray/core/dataset.py

@@ -7176,7 +7176,10 @@ def to_pandas(self) -> pd.Series | pd.DataFrame:
     def _to_dataframe(self, ordered_dims: Mapping[Any, int]):
         from xarray.core.extension_array import PandasExtensionArray
 
-        columns_in_order = [k for k in self.variables if k not in self.dims]
+        # All and only non-index arrays (whether data or coordinates) should
+        # become columns in the output DataFrame. Excluding indexes rather
+        # than dims handles the case of a MultiIndex along a single dimension.
+        columns_in_order = [k for k in self.variables if k not in self.xindexes]
         non_extension_array_columns = [
             k
             for k in columns_in_order

xarray/tests/test_dataarray.py

@@ -3524,6 +3524,18 @@ def test_to_dataframe_multiindex(self) -> None:
         assert_array_equal(index_pd.levels[1], ["a", "b"])
         assert_array_equal(index_pd.levels[2], [5, 6, 7])
 
+        # test converting a dataframe MultiIndexed along a single dimension
+        mindex_single = pd.MultiIndex.from_product(
+            [list(range(6)), list("ab")], names=["A", "B"]
+        )
+
+        arr_multi_single = DataArray(
+            arr_np.flatten(), [("MI", mindex_single)], dims="MI", name="test"
+        )
+        actual_df = arr_multi_single.to_dataframe()
+        expected_df = arr_multi_single.to_series().to_frame()
+        assert expected_df.equals(actual_df)
+
     def test_to_dataframe_0length(self) -> None:
         # regression test for #3008
         arr_np = np.random.randn(4, 0)

xarray/tests/test_dataset.py

@@ -5120,7 +5120,6 @@ def test_to_and_from_dataframe(self) -> None:
         # from_dataframe attempts to broadcast across because it doesn't know better, so cat must be converted
         ds["cat"] = (("x", "y"), np.stack((ds["cat"].to_numpy(), ds["cat"].to_numpy())))
         assert_identical(ds.assign_coords(x=[0, 1]), Dataset.from_dataframe(actual))
-
         # Check multiindex reordering
         new_order = ["x", "y"]
         # revert broadcasting fix above for 1d arrays
@@ -5154,6 +5153,41 @@ def test_to_and_from_dataframe(self) -> None:
         ):
             ds.to_dataframe(dim_order=invalid_order)
 
+        # test a case with a MultiIndex along a single dimension
+        data_dict = dict(
+            x=[1, 2, 1, 2, 1], y=["a", "a", "b", "b", "b"], z=[5, 10, 15, 20, 25]
+        )
+        data_dict_w_dims = {k: ("single_dim", v) for k, v in data_dict.items()}
+
+        # Dataset multi-indexed along "single_dim" by "x" and "y"
+        ds = Dataset(data_dict_w_dims).set_coords(["x", "y"]).set_xindex(["x", "y"])
+        expected = pd.DataFrame(data_dict).set_index(["x", "y"])
+        actual = ds.to_dataframe()
+        assert expected.equals(actual)
+        # should be possible to reset index, as there should be no duplication
+        # between index and columns, and dataframes should still be equal
+        assert expected.reset_index().equals(actual.reset_index())
+
+        # MultiIndex deduplication should not affect other coordinates.
+        mindex_single = pd.MultiIndex.from_product(
+            [list(range(6)), list("ab")], names=["A", "B"]
+        )
+        ds = DataArray(
+            range(12), [("MI", mindex_single)], dims="MI", name="test"
+        )._to_dataset_whole()
+        ds.coords["C"] = "a single value"
+        ds.coords["D"] = ds.coords["A"] ** 2
+        expected = pd.DataFrame(
+            dict(
+                test=range(12),
+                C="a single value",
+                D=[0, 0, 1, 1, 4, 4, 9, 9, 16, 16, 25, 25],
+            )
+        ).set_index(mindex_single)
+        actual = ds.to_dataframe()
+        assert expected.equals(actual)
+        assert expected.reset_index().equals(actual.reset_index())
+
         # check pathological cases
         df = pd.DataFrame([1])
         actual_ds = Dataset.from_dataframe(df)