[fetchmail]duplicate messages

Hello,

New to fetchmail, I found it great except that
with keep option, fetchmail retreives all messages 
each time I run it !  So I get many duplicate messages
in my mailbox.  I tried many options, looked at faq
but did not find any answer ?

my config is :

defaults
    no fetchall
    no rewrite
    keep
poll mail.myisp.com proto pop3 port 110
        user "my_login" with pass "my_pass" is "my_local_login" here

I tried both 5.0.9 and 5.9.12 version

Any idea ?

Thanks for help 
Philippe