reindex from a duplicate axis: inconsistent behaviour
The behaviour below occurs in version `'0.15.1'`.

When a series has a duplicate index, the method `reindex` will raise an exception, unless the index passed to `reindex` is identical to the series' index.

I propose that when a series has a duplicate index, the method `reindex` should always raise an exception, because when a series with a duplicate index is to be conformed to a new index, the intended behaviour is _always_ ambiguous.

This issue applies to the methods `reindex_like` and `reindex_axis` too.

Examples of current behaviour:

(a)

``` python
>>> pd.Series([1, 2, 3], index=['a', 'b', 'b']).reindex(['a', 'b'])
ValueError: cannot reindex from a duplicate axis
```

(b)

``` python
>>> pd.Series([1, 2, 3], index=['a', 'b', 'b']).reindex(['a', 'b', 'b'])
a    1
b    2
b    3
dtype: int64
```

The exception message in (a) implies that (b) should raise; but it doesn't.
