overflow bug in multi-index when checking for duplicates
closes https://github.com/pydata/pandas/issues/9075

@behzadnouri thanks!
 the tests break on python 3.2 (we still support this?) due to the use of the `u'..'` syntax

See the extra builds below: https://travis-ci.org/pydata/pandas/builds/44414725
 just need to do (technically we do support) - it's not hard to maintain 
though if others are dropping 3.2? 

u('value') instead of u'value'
