Duplicate NaNs when using set() to a Pandas series
#### Code Sample

```python
import numpy as np
import pandas as pd
S = pd.Series([np.nan,1.5,np.nan,2,np.nan,np.nan,np.nan,1,1,2,2,2,3])
print S; print; print set(S)
```
#### Problem description

The NaNs show up multiple times in set(S), which is not a problem if

```python
S = [np.nan,1.5,np.nan,2,np.nan,np.nan,np.nan,1,1,2,2,2,3]
```

is used instead. The multiple NaNs in set(S) also become very slow to run if

```python
S = pd.Series([np.nan,1.5,np.nan,2,np.nan,np.nan,np.nan,1,1,2,2,2,3] + [np.nan] * 100000)
```

is used. Without pd.Series it's a flash of a second.
