nicer error message on duplicate columns names

[jreback]

pandas-dev/pandas#15838 (comment)

In [9]: fastparquet.__version__
Out[9]: '0.0.5'

In [10]: fastparquet.write?

In [11]: df = pd.DataFrame(np.arange(12).reshape(4, 3), columns=list('aaa'))

In [13]: fastparquet.write('foo', df)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-13-030e6747feb8> in <module>()
----> 1 fastparquet.write('foo', df)

/Users/jreback/miniconda3/envs/pandas/lib/python3.6/site-packages/fastparquet/writer.py in write(filename, data, row_group_offsets, compression, file_scheme, open_with, mkdirs, has_nulls, write_index, partition_on, fixed_text, append, object_encoding, times)
    747     fmd = make_metadata(data, has_nulls=has_nulls, ignore_columns=ignore,
    748                         fixed_text=fixed_text, object_encoding=object_encoding,
--> 749                         times=times)
    750 
    751     if file_scheme == 'simple':

/Users/jreback/miniconda3/envs/pandas/lib/python3.6/site-packages/fastparquet/writer.py in make_metadata(data, has_nulls, ignore_columns, fixed_text, object_encoding, times)
    608                      object_encoding.get(column, None))
    609         fixed = None if fixed_text is None else fixed_text.get(column, None)
--> 610         if str(data[column].dtype) == 'category':
    611             se, type = find_type(data[column].cat.categories,
    612                                  fixed_text=fixed, object_encoding=oencoding)

/Users/jreback/pandas/pandas/core/generic.py in __getattr__(self, name)
   2888             if name in self._info_axis:
   2889                 return self[name]
-> 2890             return object.__getattribute__(self, name)
   2891 
   2892     def __setattr__(self, name, value):

AttributeError: 'DataFrame' object has no attribute 'dtype'

[jreback]

you can do df.columns.is_unique to check this FYI