BUG: pd.cut with duplicate index Series lowest included
- [x] closes #42185
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Hi @jreback , apologies, I am new to open source contributions.
I have run the tests. Below is a screenshot. I am not very sure if (and how) my change caused these failures.

![Screenshot 2021-07-07 at 8 50 43 PM](https://user-images.githubusercontent.com/6472290/124786611-7c989f00-df65-11eb-9291-e4973866c0ff.png)
 we have extensive docummentation on how to do changes: https://pandas.pydata.org/pandas-docs/dev/development/index.html

you need to add tests for your scenario and not break anything else.