In theory, the following should also work, but Perl doesn't like it one bit:

(sort{length$b<=>length$a}pop=~/.*(?(?{$&=~'(.).*\1'})^)/gs)[0]
[download]

update: blech, actually in theory it wouldn't work since i'm not using a zero-width assertion for remainder of the matched strings, but it's still interesting that it segfaults Perl.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print