fetchmail misbehaviour

[Robert S <robert.spam.me.senseless@gmail.com>]

I have been fetchmail/cron to retrieve messages from my ISP's
mailserver (POP).  Messages are being left on the server.  This system
has been running for at least a year without a glitch.  Last night I
found over 2000 messages in my inbox - for some reason fetchmail has
started not recognising messages as being "read" and is retrieving them
every 30 minutes.  I recently upgraded clamav, but the fetchmail binary
and its config files have not changed.  My configuration:

me@debian:~$ cat .fetchmailrc
poll pop.******.com.au
       proto pop3
       user "****"
       pass "*******"
       keep

me@debian:~$ crontab -l
0,30 * * * *    /usr/bin/fetchmail -s &> /dev/null

Can somebody explain why this has occurred?  I seem to remember having
this problem a number of years ago.  I'm beginning to think that using
fetchmail with the "keep" option is not a good idea.