Double email with Outlook 2007

Virtualmin Theme version: 6.6
Virtualmin version: 3.65
Operating system: Ubuntu Linux 6.06.1
There is a non-ripetitive problem with a customer that uses Outlook 2007.

Sometimes, 2 to 10 times, outlook shows the same email double, but on the server this email arrives one time.

Is there a bug into outlook 2007 ?
Or dovecot misconfiguration ?

Thanks.

The dovecot config file is the follow:

## Dovecot configuration file
protocols = imap pop3 pop3s
listen =
ssl_listen = *
ssl_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
ssl_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
disable_plaintext_auth = no
log_timestamp = "%Y-%m-%d %H:%M:%S "
mail_privileged_group = mail

protocol imap {
}

protocol pop3 {
pop3_uidl_format = %08Xu%08Xv
}

auth default {
mechanisms = plain

passdb pam {
}

userdb passwd {
}

user = root

}

plugin {
}