Plot uses incorrect colors when columns have the same name
It seems like the `plot` function jumbles up the colors when columns in a DataFrame have the same name:

``` python
pd.concat([pd.DataFrame({'b': [0, 1, 0], 'a': [1, 2, 3]}), pd.DataFrame({'a': [2, 4, 6]})], axis=1).plot();
```

![image](https://cloud.githubusercontent.com/assets/3666725/9942860/5770cf74-5d32-11e5-98b9-904ded128f57.png)
