Test doctests in pandas/core/reshape/concat.py
#### Code Sample, a copy-pastable example if possible

#### Problem description

Doctests in `concat.py` are being failed as below:

```
Failed example:
    pd.concat([df5, df6], verify_integrity=True)
Expected:
    Traceback (most recent call last):
        ...
    ValueError: Indexes have overlapping values: ['a']
Got:
    Traceback (most recent call last):
      File "/Applications/PyCharm.app/Contents/helpers/pycharm/docrunner.py", line 140, in __run
        compileflags, 1), test.globs)
      File "<doctest concat[21]>", line 1, in <module>
        pd.concat([df5, df6], verify_integrity=True)
      File "/usr/local/lib/python3.7/site-packages/pandas/core/reshape/concat.py", line 228, in concat
        copy=copy, sort=sort)
      File "/usr/local/lib/python3.7/site-packages/pandas/core/reshape/concat.py", line 381, in __init__
        self.new_axes = self._get_new_axes()
      File "/usr/local/lib/python3.7/site-packages/pandas/core/reshape/concat.py", line 461, in _get_new_axes
        new_axes[self.axis] = self._get_concat_axis()
      File "/usr/local/lib/python3.7/site-packages/pandas/core/reshape/concat.py", line 519, in _get_concat_axis
        self._maybe_check_integrity(concat_axis)
      File "/usr/local/lib/python3.7/site-packages/pandas/core/reshape/concat.py", line 528, in _maybe_check_integrity
        '{overlap!s}'.format(overlap=overlap))
    ValueError: Indexes have overlapping values: Index(['a'], dtype='object')
```

#### Expected Output

Doctests being passed.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3.final.0
python-bits: 64
OS: Darwin
OS-release: 18.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.4.1
pip: 19.0.3
setuptools: 40.8.0
Cython: None
numpy: 1.16.2
scipy: None
pyarrow: 0.13.0
xarray: None
IPython: None
sphinx: 2.0.1
patsy: None
dateutil: 2.8.0
pytz: 2019.1
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
