TST: Add test of DataFrame.xs() with duplicates (#13719) (#22294)

Author: nmusolino

pandas/tests/test_multilevel.py

@@ -10,7 +10,8 @@
 import numpy as np
 
 from pandas.core.index import Index, MultiIndex
-from pandas import Panel, DataFrame, Series, notna, isna, Timestamp, read_csv
+from pandas import (Panel, DataFrame, Series, notna, isna, Timestamp, concat,
+                    read_csv)
 
 from pandas.core.dtypes.common import is_float_dtype, is_integer_dtype
 import pandas.core.common as com
@@ -486,6 +487,14 @@ def test_xs_partial(self):
         expected = df.loc['foo', 'one']
         tm.assert_frame_equal(result, expected)
 
+    def test_xs_with_duplicates(self):
+        # Issue #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']