TST: Add tests for duplicated and drop_duplicates
- [x] refers to  #15752
- [x] tests added / passed
- [x] tests duplicated and drop_duplicates for period, categorical, datetimes, timedelta 
Hello @mproszewska! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:











There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-04-03 23:00:42 UTC can you merge master  thanks @mproszewska very nice