How about: 
perl -ne 'print if 1 == ++$seen{$_}' file

[download]
Incidentally I was involved in an interesting discussion of this topic not long ago. Some interesting material in there, including internal design philosophy differences between Python and Perl, how to handle very large files, and the fact that Java won. :-(

In reply to RE (tilly) 1: remove duplicate lines from input file by tilly
in thread remove duplicate lines from input file by AltBlue

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