pydata/pandas — pull request #8281 by bthyreau (2014-09-16 08:24:32+00:00)
Title: ENH groupby: more user-friendly error message (GH7511)

better error message for groupy on duplicated columns name. (Followup for branch GH8210 (Bug GH7511)). E.g.
```
ValueError: Grouper for 'A' not 1-dimensional
ValueError: Grouper for '' not 1-dimensional
```