You could modify these implementations.

A related problem I once had fun with. Process a file and only print lines once (unlike uniq no assumption of sorted input). That isn't quite what you wanted, but here was the fastest version I could come up with in Perl: 

perl -ne 'print if 1 == ++$s{$_}' < in > out

[download]
(Yes, details like where to place the plusses mattered.)

In reply to Re (tilly) 1: == uniq by tilly
in thread == uniq by northface

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