rmail produces mail duplicates when rmail-preserve-inbox is t

[Jarek Czekalski]

What I try to do is reading emails taken from POP3 server using Emacs, 
while not removing messages from server. I found a related Info section:

If `rmail-preserve-inbox' is non-`nil', then Rmail does not
clear out the inbox file when it gets new mail.  You may wish to set
this, for example, on a portable computer you use to check your mail
via POP while traveling, so that your mail will remain on the server
and you can save it later on your workstation.

Unfortunately now every time I press "g" (rmail-summary-get-new-mail) I 
get duplicates of the messages I already downloaded. Pop3 clients don't 
work this way.

Am I missing something or does Emacs?

Thanks,
Jarek