pd.concat does not work correctly
#### Code Sample, a copy-pastable example if possible

```python
df1.shape    # (21141, 59)
df2.shape    # (21141, 6)
result = pd.concat([df1, df2], axis=1, ignore_index=True)
result.shape    # (42282, 65)
```
#### Problem description

I have 2 dataframes that I try to concatenate horizontally. The method concat doesn't work: it returns a dataframe with a wrong dimension. Moreover, all column names happen to be changed to numbers going from 0 to 64...

The dataframes are created from a dataset that is a bit big so I cannot reproduce the creation code here but I can provide you with more details by e-mail.

#### Expected Output
The right dimension should be (21141, 65) and the resulting columns should be just the concatenation of df1's columns and df2's columns.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: fr
LOCALE: None.None

pandas: 0.22.0
pytest: 3.3.2
pip: 18.0
setuptools: 39.0.1
Cython: 0.27.3
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
