BUG: GroupBy().fillna() performance regression
- [x] closes #36757 
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

A performance regression was introduced with #30679 in handling grouping on a duplicate axis. The regression can be avoided by skipping the call to `get_indexer_non_unique` if the axis is unchanged (i.e. like it is in `fillna`). 

I don't know how to write a test for this fix since it is just an issue of speed. If there is a test pattern or other check that should be included please highlight and I'm happy to add it. 

In lieu of a test, running the minimal example from the issue report on the fixed branch shows the performance fix:

```
In [5]: import pandas as pd
   ...: import numpy as np
   ...: 
   ...: N = 2000
   ...: df = pd.DataFrame({"A": [1] * N, "B": [np.nan, 1.0] * (N // 2)})
   ...: df = df.sort_values("A").set_index("A")
   ...: 
   ...: %time df.groupby("A")["B"].fillna(method="ffill")
   ...:
Wall time: 1.03 ms
Out[5]:
A
1    NaN
1    1.0
1    1.0
1    1.0
1    1.0
    ...
1    1.0
1    1.0
1    1.0
1    1.0
1    1.0
Name: B, Length: 2000, dtype: float64            
```
an asv is appropriate here (or if we have one that covers just show the results) if not u can add one along the lines of what i posted  Hello @smithto1! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-10-17 22:24:29 UTC > pls add an asv or show an existing one that is caught by this

No existing one seemed to catch it so added a new one. 

`asv continuous --quick -f 1.1 -b groupby.FillNA upstream/master HEAD` 

<details>

(pandas-dev) C:\git\pandas-smithto1\asv_bench>python C:\anaconda3\envs\pandas-dev\Scripts\asv.exe continuous --quick -f 1.1 -b groupby.FillNA upstream/master HEAD
· Creating environments
· Discovering benchmarks
·· Uninstalling from conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
·· Installing 5412aaa0 <issue36757> into conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
· Running 4 total benchmarks (2 commits * 1 environments * 2 benchmarks)
[  0.00%] · For pandas commit 58dcafad <master> (round 1/2):
[  0.00%] ·· Building for conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[  0.00%] ·· Benchmarking conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 25.00%] · For pandas commit 5412aaa0 <issue36757> (round 1/2):
[ 25.00%] ·· Building for conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 25.00%] ·· Benchmarking conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 50.00%] · For pandas commit 5412aaa0 <issue36757> (round 2/2):
[ 50.00%] ·· Benchmarking conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 62.50%] ··· groupby.FillNA.time_df_ffill                                                     5.28±0ms
[ 75.00%] ··· groupby.FillNA.time_srs_ffill                                                    3.12±0ms
[ 75.00%] · For pandas commit 58dcafad <master> (round 2/2):
[ 75.00%] ·· Building for conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 75.00%] ·· Benchmarking conda-py3.8-Cython0.29.21-jinja2-matplotlib-numba-numexpr-numpy-odfpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 87.50%] ··· groupby.FillNA.time_df_ffill                                                      18.8±0s
[100.00%] ··· groupby.FillNA.time_srs_ffill                                                     17.7±0s
       before           after         ratio
     [58dcafad]       [5412aaa0]
     <master>         <issue36757>
-         18.8±0s         5.28±0ms     0.00  groupby.FillNA.time_df_ffill
-         17.7±0s         3.12±0ms     0.00  groupby.FillNA.time_srs_ffill

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.

</details> > pls show the results of the asv

```
(pandas-dev) C:\git\pandas-smithto1\asv_bench>python C:\anaconda3\envs\pandas-dev\Scripts\asv.exe continuous -f 1.1 -b groupby.FillNA upstream/master HEAD
       before           after         ratio
     [85793fb8]       [8a93e0b8]
     <issue36757~1^2>       <issue36757>
-     2.68±0.04ms      2.00±0.02ms     0.74  groupby.FillNA.time_df_bfill
-     2.68±0.01ms      1.99±0.05ms     0.74  groupby.FillNA.time_df_ffill
-     1.85±0.03ms      1.28±0.02ms     0.69  groupby.FillNA.time_srs_bfill
-     1.87±0.03ms      1.26±0.04ms     0.67  groupby.FillNA.time_srs_ffill

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE INCREASED.
``` LGTM, thanks @smithto1 fixing the issue reported by me recently so swiftly @jreback can you have another look. thanks @smithto1  @meeseeksdev backport 1.1.x > LGTM, thanks @smithto1 fixing the issue reported by me recently so swiftly

@alippai Thanks for reporting the issue. 