BUG: Concat to produce a non-unique columns when duplicates are across dtypes is fixed (GH3602_)

DOC: added non-unique index issues to v0.11.1
Merge pull request #3647 from jreback/concat_issue

BUG: (GH3602) Concat to produce a non-unique columns when duplicates are across dtypes
DOC: minor v0.11.1 editing