Merge pull request #6388 from jreback/dups_dropna

BUG: Bug in DataFrame.dropna with duplicate indices (GH6355)
DOC: add cookbook example for reading in simple binary file formats
Merge pull request #6390 from cpcloud/cookbook-read-bin-6305

DOC: add cookbook example for reading in simple binary file formats
API/CLN: add in common operations to Series/Index, refactored as a OpsMixin
         (GH4551, GH4056, GH5519)