BUG: to_csv duplicate index values

pandas/core/frame.py

@@ -1092,7 +1092,9 @@ def _helper_csvexcel(self, writer, na_rep=None, cols=None, header=True,
         if cols is None:
             cols = self.columns
 
-        series = self._series
+        series = {}
+        for k, v in self._series.iteritems():
+            series[k] = v.values
         if header:
             if index:
                 # should write something for index label
@@ -1122,15 +1124,15 @@ def _helper_csvexcel(self, writer, na_rep=None, cols=None, header=True,
                 writer.writerow(encoded_cols)
 
         nlevels = getattr(self.index, 'nlevels', 1)
-        for idx in self.index:
+        for j, idx in enumerate(self.index):
             row_fields = []
             if index:
                 if nlevels == 1:
                     row_fields = [idx]
                 else:  # handle MultiIndex
                     row_fields = list(idx)
             for i, col in enumerate(cols):
-                val = series[col].get(idx)
+                val = series[col][j]
                 if isnull(val):
                     val = na_rep
 

pandas/tests/test_frame.py

@@ -3015,6 +3015,22 @@ def test_to_csv_from_csv(self):
         recons = DataFrame.from_csv(path)
         assert_frame_equal(dm, recons)
 
+
+
+        #duplicate index
+        df = DataFrame(np.random.randn(3, 3), index=['a', 'a', 'b'],
+                       columns=['x', 'y', 'z'])
+        df.to_csv(path)
+        result = DataFrame.from_csv(path)
+        assert_frame_equal(result, df)
+
+        midx = MultiIndex.from_tuples([('A', 1, 2), ('A', 1, 2), ('B', 1, 2)])
+        df = DataFrame(np.random.randn(3, 3), index=midx,
+                       columns=['x', 'y', 'z'])
+        df.to_csv(path)
+        result = DataFrame.from_csv(path, index_col=[0, 1, 2],
+                                    parse_dates=False)
+        assert_frame_equal(result, df)
         os.remove(path)
 
     def test_to_csv_multiindex(self):

vb_suite/io_bench.py

@@ -38,3 +38,9 @@
 write_csv_standard = Benchmark("df.to_csv('__test__.csv')", setup2,
                                start_date=datetime(2011, 9, 15))
 
+#----------------------------------
+setup = common_setup + """
+df = DataFrame(np.random.randn(3000, 30))
+"""
+frame_to_csv = Benchmark("df.to_csv('__test__.csv')", setup,
+                         start_date=datetime(2011, 1, 1))