BUG: DataFrame.stack does not work when MultiIndex had duplicated names
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---


#### Code Sample, a copy-pastable example

`data.csv`:
```csv
    , Q1, Q1, Q1, Q2, Q2, Q2
    , V1, V2, V3, V1, V2, V3
Ram , 11, 19, 10, 10, 12, 14
Syam, 11, 19, 10, 10, 12, 14

```

```python
# Your code here
df = pd.read_clipboard(',', header=[0,1], index_col=0)
df.stack(0)

ValueError: The name      occurs multiple times, use a level number
```

#### Problem description

Well, I *am* using a level number, not a name, but there still is an error because the level names are duplicated: each is `    ` (4 spaces). I would expect it to work, or to have a more descriptive error, maybe upon creation of the data-frame: "MultiIndex should not have identical names".

I believe this is related to #19029 

However, interestingly `df.stack(1)` works correctly, no problems with duplicate names.

#### Expected Output
```
H2         V1   V2   V3
     H1
Ram   Q1   11   19   10
      Q2   10   12   14
Syam  Q1   11   19   10
      Q2   10   12   14
```

Solved it with:
```
df.columns.names = ['H1', 'H2']
df.stack(0) # works
```




#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.4.0
Version          : Darwin Kernel Version 19.4.0: Wed Mar  4 22:28:40 PST 2020; root:xnu-6153.101.6~15/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_UK.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.19.0
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 49.6.0.post20200814
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.2
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.14.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
