Multiple aggregations with the same name get overwritten
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
import numpy as np
from functools import partial

df = pd.DataFrame(
    np.random.randn(1000, 3), 
    index=pd.date_range('1/1/2012', freq='S', periods=1000), 
    columns=['A', 'B', 'C']
)
dfg = df.resample('3T').agg(
    {'A': [
        partial(np.quantile, q=.9999), 
        partial(np.quantile, q=.1111),
    ]}
)
```
returns
```
                            A          
                     quantile  quantile
2012-01-01 00:00:00 -1.035997 -1.035997
2012-01-01 00:03:00 -1.326052 -1.326052
2012-01-01 00:06:00 -1.267661 -1.267661
2012-01-01 00:09:00 -1.321961 -1.321961
2012-01-01 00:12:00 -1.103027 -1.103027
2012-01-01 00:15:00 -0.987614 -0.987614
```

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : 5e9fe4e0843266cb1c64bff4af70b3665a4d6a8d
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-74-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.0+1719.g5e9fe4e08.dirty
numpy            : 1.17.3
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 42.0.2.post20191201
Cython           : 0.29.14
pytest           : 5.3.2
hypothesis       : 4.55.4
sphinx           : 2.3.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.6
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : 4.8.1
bottleneck       : 1.3.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.2
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
pytest           : 5.3.2
s3fs             : 0.4.0
scipy            : 1.4.0
sqlalchemy       : 1.3.11
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : 0.14.1
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.6
numba            : 0.46.0
</details>
