BUG: GH3468 Fix assigning a new index to a duplicate index in a DataFrame would fail
ENH: support for having duplicative indices across blocks (dtypes)

BUG: fix construction of a DataFrame with duplicative indices
BUG: enabled applymap to work (and updated internals/convert to use iget) when

     using a non-unique index (GH2786 for the warning and GH3230 for applymap)

TST: test for GH2194 (which is fixed)
BUG: GH3495 change core/format/CSVFormatter.save to allow generic way of dealing

     with columns duplicate or not
PERF: allow a cache_readonly to be 'set' if allow_settings is passed on the decoration

      useful when specifiying an index that is **known** to be unique (e.g. in the case
      of a default range index)
Merge pull request #3509 from jreback/dup_columns2

BUG/CLN: Allow the BlockManager to have a non-unique items (axis 0)