BUG: pandas.dataframe.drop_duplicates  ignore_index parameter
- [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.

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

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import unittest
import pandas as pd

class Test(unittest.TestCase):
    def testError(self):
        cars = {'Brand': ['Honda Civic','Toyota Corolla','Ford Focus','Audi A4'],
                'Price': [22000,25000,27000,35000]
                }

        df = pd.DataFrame(cars, columns = ['Brand', 'Price'])
        dataError = df.drop_duplicates(ignore_index='error')
        print("Should give ValueError: \n"+
            "   ValueError: For argument ignore_index expected type bool, received type str.")

if __name__ == '__main__' :
    unittest.main()
```

#### Problem description

Hi! So I am doing white testing for school and found a tiny bug in drop_duplicates. When using the boolean parameter "ignore_index" it does not check if it is boolean or not. The "inplace" parameter is checked using the function validate_bool_kwarg(--). The "ignore_index" should be checked the same way. I have never reported a bug and I could fix it myself but I am a bit afraid to mess things up. Thank you!

#### Expected Output

ValueError: For argument "ignore_index" expected type bool, received type str.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.104-microsoft-standard
Version          : #1 SMP Wed Feb 19 06:37:35 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : None
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            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
