BUG: Intersection buggy for non-monotonic non-unique indices (GH8362)
Fixes intersection bug in the case of non-monotonic non-unique indexes. 
closes #8362 

needs a test (e.g. your example from the issue)
 also pls add a release note in v0.15.0.txt
 Okay I think it's fixed now.
 looks good
pls squash and rebase 
ping in green
 merged via https://github.com/pydata/pandas/commit/68fdacd12e7963706f0ac1b859058bddb6b814bd

thanks!
