BUG: non unique indexing via loc broken
This throws a weird error

```
df = DataFrame({'A' : [1,2,3,4], 'B' : [3, 4, 5, 6]}, index = [0,1,0,1]) 
df.loc[1:]
```

https://groups.google.com/forum/?fromgroups#!topic/pydata/zTm2No0crYs
