Series.loc with MultiIndex does not handle repeated keys properly
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
s = pd.Series([1, 2], pd.MultiIndex.from_arrays([['a', 'b']]))
print(s.loc[['a', 'a', 'b', 'b']])
```

#### Problem description

This prints the full series, not the series with repeated elements:
```
a    1
b    2
dtype: int64
```

#### Expected Output

```
a    1
a    1
b    2
b    2
dtype: int64
```

An easy work around is to use `s.reindex(['a', 'a', 'b', 'b'])` instead. But `loc` and `reindex` should return the same thing when all labels are found.

#### Output of ``pd.show_versions()``

pandas 0.22.0

I think this behavior is present on earlier versions of pandas, too, but previously it was very difficult to construct a MultiIndex with only a single level, because they were automatically turned in a base Index.