[Dovecot] Duplicate header extension keywords

I'm running dovecot supporting imaps and pop3s in a small home-office environment on a Linux-box, kernel 2.4.33, with postfix-2.3.4, amavisd-new and spamassassin. I use Thunderbird as mail-client on Windows XP.

ever since upgrading to RC10 I every now and then find these error messages in doveocot.log: Error: IMAP(rainer): Corrupted index file /home/rainer/Maildir/.software.Dovecot/dovecot.index: Duplicate header extension keywords

and close by:

Error: IMAP(rainer): Fixed index file /home/rainer/Maildir/.software.Dovecot/dovecot.index: first_recent_uid_lowwater 1952 -> 1951

Misconfiguration or where is the problem?

-- in diesem Sinne, Rainer