SELECT key_number, dateadded,
  IF (action=@last_action AND key_number=@last_key,"ERROR","OK"), 
  @last_action := action, @last_key := key_number
  FROM ( SELECT *
    FROM keysafe
    ORDER BY key_number, dateadded
  ) as x

##</code><code>##

$dbh->do('SET @last_action := "UNKNOWN",@last_key="UNKNOWN"');

##</code><code>##

my $sql = 'SELECT key_number,action,dateadded
           FROM keysafe
           ORDER BY key_number, dateadded';
my $sth = $dbh->prepare($sql);
$sth->execute();
my ($last_key,$last_action);
while (my ($key,$action,$date) = $sth->fetchrow_array){
  my $status = 'OK';
  if (($key eq $last_key) && ($action eq $last_action)){
    $status = 'ERROR';
  };
  print "$key,$action,$date,$status\n";
  $last_key    = $key;
  $last_action = $action;
}