Unstable hashtable / duplicated algo for object dtype
From a flaky test in geopandas, I observed the following behaviour:

```
In [1]: pd.__version__
Out[1]: '0.25.0.dev0+791.gf0919f272'

In [2]: from shapely.geometry import Point 

In [3]: a = np.array([Point(1, 1), Point(1, 1)], dtype=object) 

In [4]: pd.Series(a).duplicated()
Out[4]: 
0    False
1     True
dtype: bool

In [6]: print(pd.Series(a).duplicated()) 
   ...: print(pd.Series(a).duplicated())
0    False
1     True
dtype: bool
0    False
1    False
dtype: bool
```

So you see that sometimes it works, sometimes it does not work.

I am also not fully sure how the object hashtable works (assuming `duplicated` uses the hashtable), as the shapely Point objects are not hashable:

```
In [9]: pd.Series(a).unique()
...
TypeError: unhashable type: 'Point'
```