Labels mismatch when adding series with repeated index values in 0.19rc1
This showed up in the dask.dataframe test suite when testing against the 0.19 release candidate.  I'm unsure if this was an intended change or not:
### Pandas 0.18

``` python
In [1]: import pandas as pd

In [2]: a = pd.Series([1, 2], index=[1, 1])

In [3]: b = pd.Series([10, 10], index=[1, 2])

In [4]: a
Out[4]: 
1    1
1    2
dtype: int64

In [5]: b
Out[5]: 
1    10
2    10
dtype: int64

In [6]: a + b
Out[6]: 
1    11.0
1    12.0
2     NaN
dtype: float64
```
### Pandas 0.19rc1

``` python
In [1]: import pandas as pd

In [2]: a = pd.Series([1, 2], index=[1, 1])

In [3]: b = pd.Series([10, 10], index=[1, 2])

In [4]: a
Out[4]: 
1    1
1    2
dtype: int64

In [5]: b
Out[5]: 
1    10
2    10
dtype: int64

In [6]: a + b
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-f96fb8f649b6> in <module>()
----> 1 a + b

/home/mrocklin/Software/anaconda/lib/python3.5/site-packages/pandas/core/ops.py in wrapper(left, right, name, na_op)
    671             return NotImplemented
    672 
--> 673         left, right = _align_method_SERIES(left, right)
    674 
    675         converted = _Op.get_op(left, right, name, na_op)

/home/mrocklin/Software/anaconda/lib/python3.5/site-packages/pandas/core/ops.py in _align_method_SERIES(left, right, align_asobject)
    613                                                 return_indexers=True)
    614             # if DatetimeIndex have different tz, convert to UTC
--> 615             left.index = index
    616             right.index = index
    617 

/home/mrocklin/Software/anaconda/lib/python3.5/site-packages/pandas/core/generic.py in __setattr__(self, name, value)
   2754         try:
   2755             object.__getattribute__(self, name)
-> 2756             return object.__setattr__(self, name, value)
   2757         except AttributeError:
   2758             pass

pandas/src/properties.pyx in pandas.lib.AxisProperty.__set__ (pandas/lib.c:44336)()

/home/mrocklin/Software/anaconda/lib/python3.5/site-packages/pandas/core/series.py in _set_axis(self, axis, labels, fastpath)
    306         object.__setattr__(self, '_index', labels)
    307         if not fastpath:
--> 308             self._data.set_axis(axis, labels)
    309 
    310     def _set_subtyp(self, is_all_dates):

/home/mrocklin/Software/anaconda/lib/python3.5/site-packages/pandas/core/internals.py in set_axis(self, axis, new_labels)
   2774             raise ValueError('Length mismatch: Expected axis has %d elements, '
   2775                              'new values have %d elements' %
-> 2776                              (old_len, new_len))
   2777 
   2778         self.axes[axis] = new_labels

ValueError: Length mismatch: Expected axis has 3 elements, new values have 5 elements
```
