"invalid dtype determination in get_concat_dtype" when concating dfs with certain columns
#### Code Sample, a copy-pastable example if possible
There might be a simpler minimal example, but I was already really struggeling to identify this problem and to find this example. The problem seems to be related to strings reappearing in different positions of the tuples, different length tuples and unequal sets of `columns`.

(btw. I'm aware of MultiIndex, I would like to convert the Index to MultiIndex after the concatenation)

```python
items_a = [("b","e","c","a","b"),
         ("e","e","c","a","c"),
         ("e","a","c","a","d"),
         ("b","a","b","e"),
         ("e","b","a"),
         ("e","c","c","a")]
items_b = [("b","e","c","a","b"),
         ("a","a","d","b","d"),
         ("a","b","d","b","e"),
         ("c","b","c","a"),
         ("a","c","b"),
         ("a","d","d","b")]
df1=pd.DataFrame([range(6)], columns=items_a)
df2=pd.DataFrame([range(6)], columns=items_b)
pd.concat([df1, df2])
```

#### Problem description
This yields 
```
AssertionError: invalid dtype determination in get_concat_dtype
```

#### Expected Output
Something similar to
```python
df1.columns = [str(c) for c in df1.columns]
df2.columns = [str(c) for c in df2.columns]
pd.concat([df1, df2])
```

#### Output of ``pd.show_versions()``
(same result with pandas=0.17.1)
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-116-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.utf8
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.0.dev0+38.g6552718
pytest: 2.8.7
pip: 9.0.1
setuptools: 20.7.0
Cython: 0.23.4
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: 1.3.6
patsy: 0.4.1
dateutil: 2.7.2
pytz: 2018.3
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: 0.7.3
lxml: 3.5.0
bs4: 4.4.1
html5lib: 0.9999999
sqlalchemy: 1.0.11
pymysql: None
psycopg2: 2.6.1 (dt dec mx pq3 ext lo64)
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
