value assignment bug for multi-indexed dataframes in v1.0.0rc0
#### Problem Description
in new pre-release 1.0.0.rc0, sliced value assignment for multi-indexed dataframe with duplicated datetime indices would fail

```python
import pandas as pd
x = pd.DataFrame([[1,2,3],['a','a','c']],index=['c1','index2'],
                 columns=pd.to_datetime(['20190101','20190101','20190102'])).T
x['c2'] = np.nan
x.index.name = 'index1'
x.set_index(['index2'],append=True,inplace=True)


In [0]: x
Out[0]:
                 c1  c2
index1   index2       
20190101 a       1  NaN
         a       2  NaN
20190102 c       3  NaN


x.loc[(pd.to_datetime('20190101'),'a'),'c2'] = 1.0     # ok
x.loc[[(pd.to_datetime('20190101'),'a')],'c2'] = 1.0   # ValueError
```