TST: Add test of DataFrame.xs() with duplicates (pandas-dev#13719)

Author: Nicholas Musolino

pandas/tests/test_multilevel.py

@@ -10,7 +10,7 @@
 import numpy as np
 
 from pandas.core.index import Index, MultiIndex
-from pandas import Panel, DataFrame, Series, notna, isna, Timestamp
+from pandas import Panel, DataFrame, Series, notna, isna, Timestamp, concat
 
 from pandas.core.dtypes.common import is_float_dtype, is_integer_dtype
 import pandas.core.common as com
@@ -486,6 +486,13 @@ def test_xs_partial(self):
         expected = df.loc['foo', 'one']
         tm.assert_frame_equal(result, expected)
 
+        # DataFrame with a MultiIndex containing duplicates (#13719)
+        df_dup = concat([self.frame] * 2)
+        assert not df_dup.index.is_unique
+        expected = concat([self.frame.xs('one', level='second')] * 2)
+        tm.assert_frame_equal(df_dup.xs('one', level='second'), expected)
+        tm.assert_frame_equal(df_dup.xs(['one'], level=['second']), expected)
+
     def test_xs_level(self):
         result = self.frame.xs('two', level='second')
         expected = self.frame[self.frame.index.get_level_values(1) == 'two']