Bug in timezone conversion when datetime index has duplicates?
Hello the Pandas team,

I ran this morning into some nasty bug with Pandas 18.1. 
I was not able to find whether you are aware of it or not. Here it is

```
idx=[pd.to_datetime('2012-02-27 14:23:00') , pd.to_datetime('2012-08-27 14:33:00'), pd.to_datetime('2012-02-27 14:23:00')]
test=pd.DataFrame({'A':['one','two', 'three']},index=idx)
test['alt']=test.index.tz_localize('UTC').tz_convert('US/Eastern').tz_localize(None) 
```

```
Out[9]: 
                         A                 alt
2012-02-27 14:23:00    one 2012-02-27 09:23:00
2012-08-27 14:33:00    two 2012-08-27 10:33:00
2012-02-27 14:23:00  three 2012-02-27 10:23:00 
```

where the third column is obviously wrong. What have I done wrong here? Are indices not meant to work with duplicate values? 

Thanks!
