Security?

Hello,

I am rather new to drupal but I have managed so far. I think it's excellent (I also use SPIP, Plume CMS and GuppY). I have just had a look at the log page. A few PHP lines marked with a red x intrigue me.

One seems to be a reply to one of my own posts (visitors can send comments but these have to be approved (so far I found only one of these visitor's comments in the approval queue):
=======================
Type php
Date Friday, April 28, 2006 - 6:53am
User visiteur
Location /?q=comment/reply/13
Message Duplicate entry 'filter:3:c9dca6492fdd564de171cbfb8cf87b66' for key 1 query: INSERT INTO cache (cid, data, created, expire, headers) VALUES ('filter:3:c9dca6492fdd564de171cbfb8cf87b66', '

Drupal est un excellent CMS (un logiciel qui aide à créer et à tenir un site qui bouge) — voir Les CMS - machines à faire du Web.

\n

Drupal ne donne pas une organisation académique comme le fait SPIP que j\'ai utilisé pour Libre-Fan. Il est bien adapté pour faire participer les membres d\'un site au contenu du dit-site.

\n

En revanche, la mise en page est terriblement raide, à cause des fichus tableaux intégrés à Drupal. C\'est son défaut (qu\'il partage avec d\'autres CMS) mais c\'est ce qui en rend certaines parties très faciles à modifier. Cependant, Drupal à la ligne 66 du fichier /homepages/xx/xxxxx//htdocs/includes/database.mysql.inc.
Severity error
Hostname 202.75.49.131
=====================
I never wrote this of course: "à la ligne 66 du fichier /homepages/x:xxx/htdocs/includes/database.mysql.inc."
(here I changed the actual reference as xxx
The second php thing is more intriguing since this comes from a stupid spam site.

=========================
php
Date Wednesday, May 3, 2006 - 2:19am
User visiteur
Location /?q=comment/reply/5
Message Duplicate entry 'filter:1:464467bce0d7e431c36cdc87239eafbd' for key 1 query: INSERT INTO cache (cid, data, created, expire, headers) VALUES ('filter:1:464467bce0d7e431c36cdc87239eafbd', '

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Severity error
Hostname 202.75.49.131
==================================

Finally someone has been looking for this file, several times : robot.txt
=================
Type page not found
Date Thursday, May 4, 2006 - 12:31am
User visiteur
Location /robots.txt
Message robots.txt non trouvée.
Severity warning
Hostname 65.54.188.141
========================

Could you tell me if these PHP things (error?) mean some visitors have tried to post comments, and couldn't or if my Drupal isn't provided with enough .htaccess files (i have the default one provided by Drupal). And what about the robot.txt stuff (i know what the file is for).

Your help will be much appreciated :-)

Cheers,
eeried