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.
Same comment as #32484 are you sure this doesn't lose any coverage? @WillAyd I added comments highlighting and linking the redundant tests.

Where needed I extended the existing tests in `tests/indexes/test_common.py` or `tests/series/methods/test_drop_duplicates.py` ~~can you rebase, ping on green.~~ /azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> @jreback CI is green thanks @SaturnFromTitan 

there might be other opportunities to use the `keep` fixture in the codebase as well.