BUG: concat along the index (axis=0) of two dataframes with duplicate column name fails
- [x] I have searched the [[pandas] tag](https://stackoverflow.com/questions/tagged/pandas) on StackOverflow for similar questions.

- [x] I have asked my usage related question on [StackOverflow](https://stackoverflow.com).

---

#### Question about pandas

Hi,
I have a persistent problem with concatenating multiple DataFrames with shapes:

1. (48, 5674)
2. (48, 9022)
3. (48, 7340),
4. (47, 6539)
5. (47, 10369)
6. (47, 17242)
7. (47, 19248) 
8. (47, 14282)

If I want to concatenate this, or even any part of it with 
```python
pd.concat(df_list)
```

I get the following error:
```python
Traceback (most recent call last):
  File "E:/OneDrive/Informatik Studium/KIT Master/SS20/AGD Praktikum/phase-2/1_code/MyTest.py", line 46, in <module>
    df_result = __parallelize_dataframe(func=apply_functions, df_data=df_train.copy(), config_tupels=config_tupels)
  File "E:/OneDrive/Informatik Studium/KIT Master/SS20/AGD Praktikum/phase-2/1_code/MyTest.py", line 22, in __parallelize_dataframe
    df_pool_result = pd.concat(pool_result[0:2])
  File "E:\venv\lib\site-packages\pandas\core\reshape\concat.py", line 284, in concat
    return op.get_result()
  File "E:\venv\lib\site-packages\pandas\core\reshape\concat.py", line 497, in get_result
    mgrs_indexers, self.new_axes, concat_axis=self.axis, copy=self.copy
  File "E:\venv\lib\site-packages\pandas\core\internals\managers.py", line 2016, in concatenate_block_managers
    elif is_uniform_join_units(join_units):
  File "E:\venv\lib\site-packages\pandas\core\internals\concat.py", line 388, in is_uniform_join_units
    all(not ju.is_na or ju.block.is_extension for ju in join_units)
  File "E:\venv\lib\site-packages\pandas\core\internals\concat.py", line 388, in <genexpr>
    all(not ju.is_na or ju.block.is_extension for ju in join_units)
AttributeError: 'NoneType' object has no attribute 'is_extension'

```
I found out in my research, that blocks in the join_units are sometimes None. But I don't understand why this is so...
All table entries in my DataFrames are not None/NaN.
Unfortunately I can't post the data here, because they are very extensive.
Maybe it helps to know that I split the rows in my original dataframe for multiprocessing. Afterwards I will concatenate them again, see above.
Thanks a lot!
