Better to avoid having to make multiple passes if you can. This seems to fit the spec as presented.

#! perl -slw
use strict;

my( $lo, $hi ) = ( 700, 1200 );

my %seen;

while( <DATA> ) {
    if( m[,...\s(\S+)\s\S+,..bn(\d+)]
    and $2 >= $lo and $2 <= $hi
    and not exists $seen{ $2 }
    ) {
        print "$2 - $1";
        $seen{ $2 }++;
    }
}

__DATA__
spokanebase,Wed 24Sep03 17:59:19,50bn4575,88383 bytes
spokanebase,Wed 24Sep03 17:59:17,49bn4196,88383 bytes
spokanebase,Wed 24Sep03 17:59:10,48bn4550,88383 bytes
memphisbase,Wed 24Sep03 17:59:08,27bn700,88383 bytes
memphisbase,Wed 24Sep03 17:59:03,26bn5444,88383 bytes
havrebase,Wed 24Sep03 17:58:46,20bn5285,88383 bytes
havrebase,Wed 24Sep03 17:58:41,19bn4594,88383 bytes
alliancebase,Wed 24Sep03 17:58:56,45bn5640,88383 bytes
havrebase,Wed 24Sep03 17:58:46,20bn5285,88383 bytes
havrebase,Wed 15Sep04 17:58:41,19bn1100,88383 bytes
[download]

Output:

c:\test>junk8
700 - 24Sep03
1100 - 15Sep04
[download]