Duplicate entry on Multiindex .loc set
``` python
data = data.sortlevel()
print(data.loc[("STW AU Equity", "2014-04-29"), "split"])
data.loc[("STW AU Equity", "2014-04-29"), "split"] = 1
print(data.loc[("STW AU Equity", "2014-04-29"), "split"])

# result
1.0
symbol         date      
STW AU Equity  2014-04-29    1
               2014-04-29    1
```

Is there something I'm doing wrong, or is Pandas duplicating the value instead of updating it? If there's nothing obvious - I'll break out into a self-contained example.
