Some of the mail on the server to duplicate.

[オーエフ・テック 榊]

I am using dovecot in mbox format.
I'm going to set to leave the 30-day mail to the server with the mail
client.

The other day, some of email was received duplicate When receive mail.
Mail does not arrive between Nov 4 11:09:03 and Nov 4 11:13:25.
There should be the 259 (274-15) messages at the time of Nov 4 11:09:03
on the server from the log.
But there were the 280 messages at the time of Nov 4 11:13:25 and 21
messages was a duplicate.

Do you know the cause?

/var/log/maillog
--------------------------------------------------
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): Loading modules from
directory: /usr/lib/dovecot/pop3
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): Module loaded:
/usr/lib/dovecot/pop3/drac.so
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): Effective uid=1000, gid=1000
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): [drac]: login by xxxx from
153.219.140.xxx
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): mbox:
data=/home/xxx/mail:INBOX=/var/spool/mail/xxx
Nov  4 11:08:14 xxxxxxxx dovecot: POP3(xxxx): mbox: root=/home/xxx/mail,
index=/home/xxx/mail, inbox=/var/spool/mail/xxx
Nov  4 11:09:03 xxxxxxxx dovecot: POP3(xxxx): Disconnected: Logged out
top=0/0, retr=32/209178, del=15/274, size=3554823
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): Loading modules from
directory: /usr/lib/dovecot/pop3
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): Module loaded:
/usr/lib/dovecot/pop3/drac.so
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): Effective uid=1000, gid=1000
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): [drac]: login by xxxx from
153.219.140.xxx
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): mbox:
data=/home/xxx/mail:INBOX=/var/spool/mail/xxx
Nov  4 11:13:25 xxxxxxxx dovecot: POP3(xxxx): mbox: root=/home/xxx/mail,
index=/home/xxx/mail, inbox=/var/spool/mail/xxx
Nov  4 11:13:27 xxxxxxxx dovecot: POP3(xxxx): Disconnected: Logged out
top=0/0, retr=42/279598, del=0/280, size=3346231
--------------------------------------------------

dovecot -n
--------------------------------------------------
# 1.0.7: /etc/dovecot.conf
protocols: imaps pop3s
ssl_ca_file: /etc/mail/certs/mail.xxxxxxxx.co.jp.cacert.pem
ssl_cert_file: /etc/mail/certs/mail.xxxxxxxx.co.jp.crt
ssl_key_file: /etc/mail/certs/mail.xxxxxxxx.co.jp.key
ssl_cipher_list: ALL:!ADH:!EXPORT:!SSLv2:RC4+RSA:+HIGH:+MEDIUM:!LOW
login_dir: /var/run/dovecot/login
login_executable(default): /usr/libexec/dovecot/imap-login
login_executable(imap): /usr/libexec/dovecot/imap-login
login_executable(pop3): /usr/libexec/dovecot/pop3-login
login_greeting: ready.
mail_access_groups: mail
mail_privileged_group: mail
mail_location: mbox:~/mail:INBOX=/var/spool/mail/%u
mail_debug: yes
mbox_write_locks: dotlock fcntl
mail_executable(default): /usr/libexec/dovecot/imap
mail_executable(imap): /usr/libexec/dovecot/imap
mail_executable(pop3): /usr/libexec/dovecot/rawlog /usr/libexec/dovecot/pop3
mail_plugins(default): drac mail_log
mail_plugins(imap): drac mail_log
mail_plugins(pop3): drac
mail_plugin_dir(default): /usr/lib/dovecot/imap
mail_plugin_dir(imap): /usr/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/pop3
mail_log_max_lines_per_sec: 0
pop3_reuse_xuidl(default): no
pop3_reuse_xuidl(imap): no
pop3_reuse_xuidl(pop3): yes
pop3_lock_session(default): no
pop3_lock_session(imap): no
pop3_lock_session(pop3): yes
auth default:
  passdb:
    driver: pam
  userdb:
    driver: passwd
plugin:
  drac_host: xxx.xxx.xxx.xxx
--------------------------------------------------

uname -a
--------------------------------------------------
Linux xxxxxxxx.co.jp 2.6.18-371.9.1.el5PAE #1 SMP Tue Jun 10 19:05:56
EDT 2014 i686 i686 i386 GNU/Linux
--------------------------------------------------