BUG: pd.Grouper cannot be reused in some cases 
- [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.

---

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

```python
import numpy as np 
import pandas as pd 
np.random.seed(1)
# Create one dataframe 
dr = pd.date_range("2021-01-01", "2021-08-30")
ind = np.random.randint(0,5,len(dr))
something = np.random.random(len(ind))
df = pd.DataFrame({"date":dr, "ind":ind, "something":something})

# Define what to group on 
GROUP_ON = ["ind", pd.Grouper(key="date", freq="2W")]
# Copy used if you uncomment lines below 
GROUP_ON_COPY = ["ind", pd.Grouper(key="date", freq="2W")]

# Comment either of the two lines below, and it works 
df.groupby(GROUP_ON).mean().reset_index().groupby(GROUP_ON).mean()
df.groupby(GROUP_ON).mean().reset_index()

# Use a different groupby in the second case, and it works 
#df.groupby(GROUP_ON).mean().reset_index().groupby(GROUP_ON).sum()
#df.groupby(GROUP_ON_COPY).mean().reset_index()

# Bonus points: do only one groupby first, and it works 
# df.groupby(GROUP_ON).mean().reset_index()
# df.groupby(GROUP_ON).mean().reset_index()

```

#### Problem description

In some cases it seems that pd.Grouper cannot be reused. I was not able to figure out exactly why, or if the output can sometimes corrupt. It leads to very painful debugging. 

Tested with master branch, and with python 3.8 and 3.9 

#### Expected Output

pd.Grouper should be able to be used any number of times, or should raise explicit error upon any reuse. 

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.9.4.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-19041-Microsoft
Version          : #488-Microsoft Mon Sep 01 13:43:00 PST 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.4
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.2
setuptools       : 57.0.0
Cython           : 0.29.23
pytest           : 6.2.4
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : 1.4.3
lxml.etree       : 4.6.3
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.24.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : 2021.06.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.3
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : None

</details>
