BUG: The exception is arisen when `apply` takes functions with equal names
- [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.

- [ ] (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 pandas
pdf = pandas.DataFrame([[4, 9]] * 3, columns=['A', 'B'])
pdf.apply(["sum", "sum"])
SpecificationError: Function names must be unique if there is no new column names assigned
```

#### Problem description

Could anyone please explain? Is it normal behavior?

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

<details>

INSTALLED VERSIONS
------------------
pandas           : 1.1.1
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 41.2.0
Cython           : None
pytest           : 5.4.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : 0.4.1
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.14.0
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : None
fsspec           : 0.7.3
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.4.1
sqlalchemy       : 1.3.17
tables           : 3.6.1
tabulate         : None
xarray           : 0.15.1
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
