Removing unique elements from string

(GolfScript, 15 13 characters)

:;{.;?);>?)},

GolfScript is not one of the top 20, but a codegolf without GolfScript... (run it yourself)

Previous Version: (run script)

1/:;{;\-,;,(<},

Perl, 28 24 characters (includes 1 for 'p' option)

s/./$&x(s!$&!$&!g>1)/eg

Usage:

> perl -pe 's/./$&x(s!$&!$&!g>1)/eg'
helloworld
llool

At first I thought I could do this with negative look-ahead and negative look-behind, but it turns out that negative look-behinds must have a fixed length. So I went for nested regexes instead. With thanks to mob for the $& tip.

J, 12 characters

Having entered a valid Perl answer, here's an invalid (language not in the TIOBE top 20) answer.

a=:#~1<+/@e.

Usage:

   a 'helloworld'
llool

Declares a verb a which outputs only non unique items.