ENH: Preserve .names in df.set_index(df.index)

Preserve .names in df.set_index(df.index)

Check that df.set_index(df.index) doesn't convert a MultiIndex to an Index

Handle general case of df.set_index([df.index,...])

Cleanup

Add to release notes

Add equality checks

Fix issue on 2.6

Add example to whatsnew
informative error message
Merge pull request #6535 from clarkfitzg/error_message

BUG: informative error message with Series ValueError
Merge pull request #6459 from qwhelan/set_index_names

ENH: Preserve .names in df.set_index(df.index)
BUG: Bug in setitem with a duplicate index and an alignable rhs (GH6541)
Merge pull request #6542 from jreback/dup_assign

BUG: Bug in setitem with a duplicate index and an alignable rhs (GH6541)
ENH date_range accepts timedelta as freq