Accessing Series with duplicate index returns Series, but accessing a unique index returns scalar
Suppose I have a Series with a duplicate indices:

``` python
In [109]: s = pd.Series([1, 2, 3, 4, 5], index=['a', 'a', 'a', 'a', 'b'])

In [110]: s
Out[110]:
a    1
a    2
a    3
a    4
b    5
dtype: int64
```

If I access the duplicated index, I get a Series:

``` python
In [111]: s.xs('a')
Out[111]:
a    1
a    2
a    3
a    4
dtype: int64
```

But if I access a unique index, I get a scalar:

``` python
In [112]: s.xs('b')
Out[112]: 5
```

Thus, I get different return types. This led to a really hard bug to track down.
