my %seen;
while ( my ( $user, $ip ) = next_pair() ) {
  next if $seen{ $user }{ $ip }++;
  insert( $user, $ip );
}

##</code><code>##

  next if $seen{ $user }++;

##</code><code>##

  next if $seen{ $ip }++;

##</code><code>##

  next if $seen{ uc $user }{ $ip }++;