TST: Added test for dropping index with dup cols

Author: aschade

pandas/tests/frame/test_mutate_columns.py

@@ -257,3 +257,21 @@ def test_insert_column_bug_4032(self):
         expected = DataFrame([[1.3, 1, 1.1], [2.3, 2, 2.2]],
                              columns=['c', 'a', 'b'])
         assert_frame_equal(result, expected)
+
+    data = [[1, 2, 3], [1, 2, 3]]
+
+    @pytest.mark.parametrize('actual', [
+        DataFrame(data=data, index=['a', 'a']),
+        DataFrame(data=data, index=['a', 'b']),
+        DataFrame(data=data, index=['a', 'b']).set_index([0, 1]),
+        DataFrame(data=data, index=['a', 'a']).set_index([0, 1])
+    ])
+    def test_raise_on_drop_duplicate_index(self, actual):
+
+        # issue 19186
+        level = 0 if isinstance(actual.index, MultiIndex) else None
+        with pytest.raises(ValueError):
+            actual.drop('c', level=level, axis=0)
+        expected_no_err = actual.drop('c', axis=0, level=level,
+                                      errors='ignore')
+        assert_frame_equal(expected_no_err, actual)