You would need some code to print out the array of duplicates but this will build that array.

open( FH, "filename" ) || die "no way man\n";
while ( <FH> ) {
    if ( $done{$_} ne "" ) {
        $dup++;
    } else {
        $done{$_} = 1;
        push( @dups, $_ );
    }
}
[download]

- kel -