df.sort(col) fails if col is not unique.
one more trouble case with duplicate column names
the exception could be nicer (sort ambiguous because of duplicate column or similar) or it could sort by the first? column with this name (although this would not be nice in case the column is not duplicate but has in fact different content with the same name and the user not being aware of this).

```
import pandas as pd
df = pd.DataFrame([(1,2), (3,4)], columns = ['a', 'a'])
df.sort('a')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-251abbb5c77e> in <module>()
      1 import pandas as pd
      2 df = pd.DataFrame([(1,2), (3,4)], columns = ['a', 'a'])
----> 3 df.sort('a')

/usr/local/lib/python2.7/dist-packages/pandas-0.10.0.dev_74ab638-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in sort(self, columns, column, axis, ascending, inplace)
   3062             columns = column
   3063         return self.sort_index(by=columns, axis=axis, ascending=ascending,
-> 3064                                inplace=inplace)
   3065 
   3066     def sort_index(self, axis=0, by=None, ascending=True, inplace=False):

/usr/local/lib/python2.7/dist-packages/pandas-0.10.0.dev_74ab638-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in sort_index(self, axis, by, ascending, inplace)
   3125             self._clear_item_cache()
   3126         else:
-> 3127             return self.take(indexer, axis=axis)
   3128 
   3129     def sortlevel(self, level=0, axis=0, ascending=True, inplace=False):

/usr/local/lib/python2.7/dist-packages/pandas-0.10.0.dev_74ab638-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in take(self, indices, axis)
   2853             new_values = com.take_2d(self.values,
   2854                                      com._ensure_int64(indices),
-> 2855                                      axis=axis)
   2856             if axis == 0:
   2857                 new_columns = self.columns

/usr/local/lib/python2.7/dist-packages/pandas-0.10.0.dev_74ab638-py2.7-linux-x86_64.egg/pandas/core/common.pyc in take_2d(arr, indexer, out, mask, needs_masking, axis, fill_value)
    414                 out = np.empty(out_shape, dtype=arr.dtype)
    415             take_f = _get_take2d_function(dtype_str, axis=axis)
--> 416             take_f(arr, _ensure_int64(indexer), out=out, fill_value=fill_value)
    417             return out
    418     elif dtype_str in ('float64', 'object', 'datetime64[ns]'):

/usr/local/lib/python2.7/dist-packages/pandas-0.10.0.dev_74ab638-py2.7-linux-x86_64.egg/pandas/algos.so in pandas.algos.take_2d_axis0_int64 (pandas/algos.c:73957)()

ValueError: Buffer has wrong number of dimensions (expected 1, got 2)
```
