CLN: Remove redundant tests for .duplicated and .drop_duplicates in tests/base
part of #23877
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

duplicated and drop_duplicates are already thoroughly tested in `tests/indexes` and `tests/series`. I added comments to highlight the redundancy and extended the existing test cases where needed.