[Dovecot] Slight problem with dovecot + IMAP

I'm running dovecot on my desktop with kmail as IMAP client on my laptop.

When I start kmail on the laptop it sometimes takes several minutes to "calm down". The number of messages listed in the IMAP inbox increases greatly - maybe from 2500 to 3000 - and then slowly decreases. There are sometimes a large number of duplicate messages, which kmail allows one to delete.

I don't know if this is a kmail or dovecot problem. It is quite likely there is something wrong in my setup.

Any suggestions or advice gratefully received.