Unhelpful error message when groupby() called with a column that is duplicated
The following code:

```
import pandas as pd
p1 = {'name': 'willy', 'age': 10}
p2 = {'name': 'willy', 'age': 11}
p3 = {'name': 'zoe', 'age': 10}
df = pd.DataFrame([p1, p2, p3])
df = df[['name','name','age']]
df.groupby('name')
```

gives the following error:

```
TypeError: 'DataFrame' object is not callable
```

Instead, it should say "Cannot groupby a column which exists more than once"
