BUG: Spurious matches in DataFrame.duplicated when keep=False
Fixes GH #11864

lgtm - ping when updated and green
 @evanpw can you rebase / update
 Added test loop, rebased, travis is green.
 merged via https://github.com/pydata/pandas/commit/b431f85a505a4d84e38f4d6f9ed5080775d4f31c

thanks!
