_sanitize_column now reports proper duplicate error
closes #7432
A very simple fix for GH7432. `internals.py` (line 3240) actually raises the correct exception (`ValueError: cannot reindex from a duplicate axis`) but the except: doesn't allow it to be raised. Here I've just allowed the exception itself to be raised since it's more useful to the user than the weird `TypeError` message.

Here's what the situation in [Issue 7432](https://github.com/pydata/pandas/issues/7432) now looks like:

```
In [4]: df['newcol'] = ser
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-acaa0ccb832e> in <module>()
----> 1 df['hello'] = ser

/home/rob/pandas_roblevy/pandas/core/frame.py in __setitem__(self, key, value)
   1853         else:
   1854             # set column
-> 1855             self._set_item(key, value)
   1856 
   1857     def _setitem_slice(self, key, value):

/home/rob/pandas_roblevy/pandas/core/frame.py in _set_item(self, key, value)
   1921         is_existing = key in self.columns
   1922         self._ensure_valid_index(value)
-> 1923         value = self._sanitize_column(key, value)
   1924         NDFrame._set_item(self, key, value)
   1925 

/home/rob/pandas_roblevy/pandas/core/frame.py in _sanitize_column(self, key, value)
   1964                     value = value.reindex(self.index).values
   1965                 except ValueError as e:
-> 1966                     raise e
   1967                 except:
   1968                     raise TypeError('incompatible index of inserted column '

ValueError: cannot reindex from a duplicate axis
```
 yep..looks trivial...can you rebase (so that travis will then run).
 thanks!
 the importance of testing! turns out their are some other error messages here that are effectively raising as TypeError, fixed up here: https://github.com/pydata/pandas/commit/7ef4efa306bbd8685f849c9b65070523b10ecb15
 Amazing. Next time I do a pull request I'll add a test for whatever I add. This is a good learning experience for me...
