BUG: cant modify df with duplicate index (pandas-dev#17105)

Author: Saravia Rajal

pandas/core/indexing.py

@@ -1319,7 +1319,7 @@ def _convert_to_indexer(self, obj, axis=None, is_setter=False):
                     (indexer,
                      missing) = labels.get_indexer_non_unique(objarr)
                     # 'indexer' has dupes, create 'check' using 'missing'
-                    check = np.zeros_like(objarr)
+                    check = np.zeros(len(objarr))
                     check[missing] = -1
 
                 mask = check == -1

pandas/tests/indexing/test_indexing.py

@@ -1018,3 +1018,20 @@ def test_validate_indices_high():
 def test_validate_indices_empty():
     with tm.assert_raises_regex(IndexError, "indices are out"):
         validate_indices(np.array([0, 1]), 0)
+
+
+def test_modify_with_duplicate_index():
+    trange = pd.date_range(start=pd.Timestamp(year=2017, month=1, day=1),
+                           end=pd.Timestamp(year=2017, month=1, day=5))
+
+    # insert a duplicate element to the index
+    trange = trange.insert(loc=5, item=pd.Timestamp(year=2017, month=1, day=5))
+
+    df = pd.DataFrame(0, index=trange, columns=["A", "B"])
+    bool_idx = np.array([False, False, False, False, False, True])
+
+    # modify the value for the duplicate index entry
+    df.loc[trange[bool_idx], "A"] = 7
+
+    assert df['A'][4] == 7
+    assert df['A'][5] == 7