edit: this is failing if a line is repeated more than once, see @showdead's excellent explanation

the shortest I've got so far is

    perl -lnE'say if!++$#$_'

---

I don't understand what's happening with $#$_ but seems like something I should look into, thanks :)

you could remove n as p is used and would be same no. of characters as

    perl -ne 'print if $#$_++'

you could save one more by removing space between e switch and single quote