A problem in gnus with pop

[Tang Yan]

I am a newbie so maybe this is a silly question.

I use gnus to receive my mail through pop3, but I always get duplicated
messages, that is each time I start gnus, it will fetch all the old and
new messages on the mail server.

Details is as following:

my gnus.el:
-----------------------------------
(setq user-mail-address "foo@bar"
user-full-name "foo")

(setq mail-sources '((pop :server "pop.bar"
:user "foo"
:password "!@#$%^")))

;; Leave all the mails on the server so that I can read them in
;; other place.
(setq pop3-leave-mail-on-server t)

(setq gnus-select-method '(nnml "private"))
-----------------------------------

Suppost there are totally 3 new mails on my mail server.

Then, the first time I fire up gnus I get(after subscribing):
3: mail.misc
And after I read all of them I get:
0: mail.misc
Then, I quit gnus and re-enter with M-x gnus, something strange happens
first in minibuffer:
fetching 3 mails ....
And I get:
3: mail.misc

But actually there are only 3 mails in my mailbox. Gnus fetchs them
twice.

I also have done some test. If in the first time I don't read the 3 new
mails, exiting gnus just after I get:
3: mail.misc
and re-enter gnus I will get:
6: mail.misc

So strange!!! Well If I comment the line (setq
pop3-leave-mail-on-server t), everything goes well. But I cannot read
mail in other places, ;-(

I need your help, could you give me some advice?

Thank you very much!

BHW: Yesterday I tried config imap in gnus, today I try pop. Both
failed. Is Gnus terrible for only reading mails?