Duplicated incorrectly returning unique datetime items
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.read_csv(file)
df.index = pd.to_datetime(df[['Year','Month','Day','Hour']], unit='s')
df[df.index.duplicated()].index
```

> DatetimeIndex(['2009-01-01 00:00:00', '2009-01-01 00:30:00',
>                '2009-01-01 01:00:00', '2009-01-01 01:30:00',
>                '2009-01-01 02:00:00', '2009-01-01 02:30:00',
>                '2009-01-01 03:00:00', '2009-01-01 03:30:00',
>                '2009-01-01 04:00:00', '2009-01-01 04:30:00',
>                ...
>                '2013-12-31 19:00:00', '2013-12-31 19:30:00',
>                '2013-12-31 20:00:00', '2013-12-31 20:30:00',
>                '2013-12-31 21:00:00', '2013-12-31 21:30:00',
>                '2013-12-31 22:00:00', '2013-12-31 22:30:00',
>                '2013-12-31 23:00:00', '2013-12-31 23:30:00'],
>               dtype='datetime64[ns]', length=175296, freq=None)

#### Problem description
The behaviour of `duplicated()` is returning duplicated days when the `datetime` values are in fact unique at their native temporal resolution. I would expect this function to return true duplicates.