add groupby testcase (pandas-dev#19029)

Author: guenteru

pandas/tests/groupby/test_groupby.py

@@ -1674,3 +1674,22 @@ def test_tuple_correct_keyerror():
                                                           [3, 4]]))
     with tm.assert_raises_regex(KeyError, "(7, 8)"):
         df.groupby((7, 8)).mean()
+
+
+def test_dup_index_names():
+    # duplicated index names in groupby operations should be renamed (GH 19029):
+    df = pd.DataFrame(data={'date': list(pd.date_range('5.1.2018', '5.3.2018')),
+                            'vals': list(range(3))})
+
+    mi = pd.MultiIndex.from_product([[5], [1, 2, 3]], names=['date0', 'date1'])
+    expected = pd.Series(data=list(range(3)), index=mi, name='vals')
+
+    failed = False
+    try:
+        result = df.groupby([df.date.dt.month, df.date.dt.day])['vals'].sum()
+    except ValueError as e:
+        failed = True
+
+    assert failed == False
+
+    tm.assert_series_equal(result,expected)