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)`
