BUG: allow dup indexing with Float64Index
Merge pull request #7149 from cpcloud/float64index-dup-fix-7143

BUG: allow dup indexing with Float64Index
BUG: union should not try to sort inplace because platform impls differ as to when sorting occurs for objects that cannot be compared
TST: Assert that a RuntimeWarning is triggered
TST: should not warn on PY3 bc tp_compare was removed
WRN: warn about sorting inplace for non-comparable types
TST/CLN: make sure RuntimeWarning is raised
WRN: warn on copying sort
BUG/TST: DatetimeIndex has join precedence over Int64Index
BUG/DUH: don't copy just check the first value

of each self and other throw a type error on ``<``,
if not we can sort inplace on the concat, otherwise we issue a warn and make no
changes