BUG: fix error for reset_index() with index.name in MultiIndex columns (#16126)

closes #16120

Author: toobaz

doc/source/whatsnew/v0.20.0.txt

@@ -1623,6 +1623,7 @@ Indexing
 - Bug in ``Series.iloc`` where a ``Categorical`` object for list-like indexes input was returned, where a ``Series`` was expected. (:issue:`14580`)
 - Bug in ``DataFrame.isin`` comparing datetimelike to empty frame (:issue:`15473`)
 - Bug in ``.reset_index()`` when an all ``NaN`` level of a ``MultiIndex`` would fail (:issue:`6322`)
+- Bug in ``.reset_index()`` when raising error for index name already present in ``MultiIndex`` columns (:issue:`16120`)
 - Bug in creating a ``MultiIndex`` with tuples and not passing a list of names; this will now raise ``ValueError`` (:issue:`15110`)
 - Bug in the HTML display with with a ``MultiIndex`` and truncation (:issue:`14882`)
 - Bug in the display of ``.info()`` where a qualifier (+) would always be displayed with a ``MultiIndex`` that contains only non-strings (:issue:`15245`)

pandas/core/internals.py

@@ -3807,7 +3807,7 @@ def insert(self, loc, item, value, allow_duplicates=False):
         """
         if not allow_duplicates and item in self.items:
             # Should this be a different kind of error??
-            raise ValueError('cannot insert %s, already exists' % item)
+            raise ValueError('cannot insert {}, already exists'.format(item))
 
         if not isinstance(loc, int):
             raise TypeError("loc must be int")

pandas/tests/test_multilevel.py

@@ -2235,6 +2235,22 @@ def test_reset_index_period(self):
         }, columns=['month', 'feature', 'a'])
         tm.assert_frame_equal(df.reset_index(), expected)
 
+    def test_reset_index_multiindex_columns(self):
+        levels = [['A', ''], ['B', 'b']]
+        df = pd.DataFrame([[0, 2], [1, 3]],
+                          columns=pd.MultiIndex.from_tuples(levels))
+        expected = df.copy()
+        df.index.name = 'A'
+        result = df[['B']].reset_index()
+        tm.assert_frame_equal(result, expected)
+
+        # GH 16120
+        # already existing column
+        with tm.assertRaisesRegexp(ValueError,
+                                   ("cannot insert \('A', ''\), "
+                                    "already exists")):
+            df.reset_index()
+
     def test_set_index_period(self):
         # GH 6631
         df = DataFrame(np.random.random(6))