Duplicate messages

[absolutely_free at libero.it]

Hi,
I just moved mail spool to a different network storage.
Now, several users are complaining about duplicate message that are fetched by their clients (Outlook, Microsoft Outlook).
What is the reason?

This is my conf:

# dovecot -n
# 2.0.9: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-71.el6.x86_64 x86_64 CentOS release 6.6 (Final)
auth_mechanisms = plain login digest-md5 cram-md5
disable_plaintext_auth = no
first_valid_gid = 89
first_valid_uid = 89
mail_gid = 89
mail_location = maildir:/coraid-s2l2/domains
mail_uid = 89
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date imapflags notify
mbox_write_locks = fcntl
namespace {
  inbox = yes
  location =
  prefix = INBOX.
  separator = .
  type = private
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
passdb {
  args = /etc/dovecot/dovecot-sql-crypt.conf.ext
  driver = sql
}
plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
  sieve_extensions = +notify +imapflags
  sieve_max_script_size = 1M
}
protocols = imap pop3 lmtp sieve
ssl_cert = </etc/nginx/ssl/wildcard.server.net.crt
ssl_key = </etc/nginx/ssl/wildcard.server.net.key
userdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocol lmtp {
  mail_plugins =
}
protocol lda {
  mail_plugins =
}