BUG: DataFrame subplot with duplicated columns output incorrect result
When `DataFrame` has duplicated column name, each subplot will contain all the lines with same name.

```
import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.rand(5, 5), columns=list('aaaaa'))
df.plot(subplots=True)
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/9609047/7acba4d0-510c-11e5-872d-1fcc1fcf8ce6.png)
