ENH: Add duplicated/drop_duplicates to Index

Author: sinhrks

doc/source/v0.15.0.txt

@@ -344,7 +344,6 @@ API changes
 - ``Series.to_csv()`` now returns a string when ``path=None``, matching the behaviour of
     ``DataFrame.to_csv()`` (:issue:`8215`).
 
-
 .. _whatsnew_0150.index_set_ops:
 
 - The Index set operations ``+`` and ``-`` were deprecated in order to provide these for numeric type operations on certain index types. ``+`` can be replace by ``.union()`` or ``|``, and ``-`` by ``.difference()``. Further the method name ``Index.diff()`` is deprecated and can be replaced by ``Index.difference()`` (:issue:`8226`)