DataFrame.ix losing row ordering when index has duplicates

``` python
import pandas as pd

ind = ['A', 'A', 'B', 'C']i
df = pd.DataFrame({'test':range(len(ind))}, index=ind)

rows = ['C', 'B']
res = df.ix[rows]
assert rows == list(res.index) # fails
```

The problem is that the resulting DataFrame keeps the ordering of the `df.index` and not the `rows` key. You'll notice that the `rows` key doesn't reference a duplicate value. 
