BUG: Stack creates duplicate column
```
import numpy as np
import pandas as pd

df = pd.DataFrame(
    np.zeros([1, 5]), 
    columns=pd.MultiIndex.from_tuples(
        [
            (0, None, None), 
            (0, 0, 0),
            (0, 0, 1),
            (0, 1, 0),
            (0, 1, 1),
        ],
    )
)
```
![image](https://user-images.githubusercontent.com/16045385/106360891-efe11980-6312-11eb-8682-a5ed546b8d10.png)
```
df.stack(2)
```
![image](https://user-images.githubusercontent.com/16045385/106360914-05564380-6313-11eb-8ebb-b5a35a74f4b5.png)


```

INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.6.6.final.0
pandas           : 1.1.5
numpy            : 1.17.4
```