Probably the easiest way to do this would be to change the following line:

  $spair{$si."-".$sn} = $prod;
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to something along the lines of:

  if ((! defined($spair{$si."-".$sn})) and 
      (! defined($spair{$sn."-".$si}))) {
    $spair{$si."-".$sn} = $prod;
  }
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This will only add the entry for $si."-".$sn if there is no entry currently in %spair for either $si."-".$sn nor $sn."-".$si.

Hope that helps.