Asset Frame with duplicate index
- [ ] closes #26494
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Hey guys, I started working on a way to make testing.assert_frame_equal accept an argument so it can test equals a frame with duplicate index just like the issue I marked to close here. For this I had to make a couple of changes in some functions and I think I got to the point that I should ask for a better direction and if this is valid for the project.

The error pointed in the issue is definitely not the best message for this case. I got to the point to not get that error by making clear there is a duplicate index. But I got stuck in this error now:

`ValueError: Shape of passed values is (5, 1), indices imply (3, 1)`

Hello @gabriellm1! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/frame.py`](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/core/frame.py):

> [Line 3743:89](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/core/frame.py#L3743): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (97 > 88 characters)

* In the file [`pandas/core/generic.py`](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/core/generic.py):

> [Line 3832:89](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/core/generic.py#L3832): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (104 > 88 characters)

* In the file [`pandas/util/testing.py`](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/util/testing.py):

> [Line 1348:46](https://github.com/pandas-dev/pandas/blob/471c7cec3cda92fb6d23336213aa7bfe982b0c56/pandas/util/testing.py#L1348): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','

 Hmm not sure I am on board with making this change. The point of `check_like` is to ignore position but align on label. If labels are duplicated how would that alignment work? Hmmm now I see the problem. There would be multiple rows to align and check if equals. In a big dataframe that alignment solution would probably take too long to run. So probably there isn't a solution for this right? Yea I think the alignment here as to what should be checked is ambiguous. Appreciate the PR as always but I don't think this one is worth pursuing (if anyone disagrees feel free to reopen)