Can't use same function twice with named aggregation
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
from functools import partial

quant50 = partial(np.percentile, q=50)
quant70 = partial(np.percentile, q=70)

test = pd.DataFrame({'col1': ['a', 'a', 'b', 'b', 'b'], 'col2': [1,2,3,4,5]})

test.groupby('col1').agg(
    quantile_50=('col2', quant50),
    quantile_70=('col2', quant70)
)
```
#### Problem description

I'm trying to calculate multiple percentiles, I get the following error: "Function names must be unique, found multiple named percentile"

It's an old problem https://github.com/pandas-dev/pandas/issues/7186 but I assumed that it should've been solved with the named aggregations. Now there is no need to deduce the column name from the function name, the column name was specified explicitly.

#### Expected Output

column | quantile_50 | quantile_70
a | 1.5 | 2
b | 4.0 | 3

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.1.11-200.fc29.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2017.3
dateutil         : 2.7.2
pip              : 19.1.1
setuptools       : 39.1.0
Cython           : 0.28.1
pytest           : 3.3.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : 0.4.0
xlsxwriter       : None
lxml.etree       : 4.3.3
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 6.3.1
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.3
matplotlib       : 2.2.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.9.0
pytables         : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.1.18
tables           : None
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : None

</details>
