REGR: DataFrame.agg with axis=1, EA dtype, and duplicate index
- [x] closes #42380
- [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

The issue in transpose was a bug that was introduced in 1.0.0, but induced a regression in 1.3.0 due to its use in `DataFrame.agg`.