Why .sort_values() on column containing same values shuffles entire dataframe ?
```
df = pd.DataFrame({
    'col1': ['A', 'A', 'A', 'A', 'A', 'A']*15,
    'col2': [1.0, 1.0, 1.0, 1.0, 1.0, 1.0]*15,
    'col3': [0, 1, 9, 4, 2, 3]*15,
    'col4': ['a', 'B', 'c', 'D', 'e', 'F']*15
})
df.sort_values('col2', ascending=True)
```

Why this operation disorders the entire dataframe ? The same will be produced if the function will be applied for the col1.

![image](https://user-images.githubusercontent.com/16267961/108284112-e403ad00-7184-11eb-97b2-7e26e06d1fd0.png)

