iat, iloc don't work with no unique index
```
import pandas as pd

s = pd.Series(range(5), index=[1,1,2,2,3])
s.iat[2]
```

which returns `array([2, 3], dtype=int64)`, I think the result should be `2`.

`s.iloc[2]` works, but `s.iloc[[2, 3]]` raise error.
