BUG: Bug in assignment with duplicate columns in a frame where the locations

     are a slice (e.g. next to each other) (GH6120)

Author: jreback

doc/source/release.rst

@@ -157,6 +157,8 @@ Bug Fixes
   - Bug with insert of strings into DatetimeIndex (:issue:`5818`)
   - Fixed unicode bug in to_html/HTML repr (:issue:`6098`)
   - Fixed missing arg validation in get_options_data (:issue:`6105`)
+  - Bug in assignment with duplicate columns in a frame where the locations
+    are a slice (e.g. next to each other) (:issue:`6120`)
 
 pandas 0.13.0
 -------------

pandas/core/internals.py

@@ -2969,7 +2969,7 @@ def _set_item(item, arr):
                 # we are inserting one by one, so the index can go from unique
                 # to non-unique during the loop, need to have _ref_locs defined
                 # at all times
-                if np.isscalar(item) and com.is_list_like(loc):
+                if np.isscalar(item) and (com.is_list_like(loc) or isinstance(loc, slice)):
 
                     # first delete from all blocks
                     self.delete(item)

pandas/tests/test_frame.py

@@ -3229,6 +3229,21 @@ def check(result, expected=None):
             result = getattr(df,op)(df)
             check(result,expected)
 
+        # multiple assignments that change dtypes
+        # the location indexer is a slice
+        # GH 6120
+        df = DataFrame(np.random.randn(5,2), columns=['that', 'that'])
+        expected = DataFrame(1.0, index=range(5), columns=['that', 'that'])
+
+        df['that'] = 1.0
+        check(df, expected)
+
+        df = DataFrame(np.random.rand(5,2), columns=['that', 'that'])
+        expected = DataFrame(1, index=range(5), columns=['that', 'that'])
+
+        df['that'] = 1
+        check(df, expected)
+
     def test_column_dups_indexing(self):
 
         def check(result, expected=None):