Added test to make sure a ValueError is thrown

Author: cmazzullo

pandas/tests/indexes/test_multi.py

@@ -2985,3 +2985,14 @@ def test_nan_stays_float(self):
         assert pd.isna(df0.index.get_level_values(1)).all()
         # the following failed in 0.14.1
         assert pd.isna(dfm.index.get_level_values(1)[:-1]).all()
+
+    def test_duplicate_multiindex_labels(self):
+        # GH 17464
+        # Make sure that a MultiIndex with duplicate levels throws a ValueError
+        with pytest.raises(ValueError):
+            ind = pd.MultiIndex([['A'] * 10, range(10)], [[0] * 10, range(10)])
+        # And that using set_levels with duplicate levels fails
+        ind = MultiIndex.from_arrays([['A', 'A', 'B', 'B', 'B'], [1, 2, 1, 2, 3]])
+        with pytest.raises(ValueError):
+            ind.set_levels([['A', 'B', 'A', 'A', 'B'], [2, 1, 3, -2, 5]],
+                           inplace=True)