Random duplicated emails

[Francis]

Hello,

I migrated our mail infrastructure to Dovecot on Ubuntu 18.04 some months
ago. It works fine, but recently some users told me that they sometime
receive duplicated emails. Same email content, same headers including
message-id.

I'm using two dovecot servers on two sites. Both server are in cluster. We
don't use shared folders. All users that reported this issue so far are
using the same server instance. The problematic  emails are coming from
local users on that instance too. The examples they given to me was emails
with many recipients (To/CC). A specific message can be received twice (or
more) by recipient A but only once by recipient B. I didn't see anything in
the logs about sieve rules that redirect emails to others recipients.

Where should I look to diagnostic this issue?

Thanks.

Server config:
# 2.2.33.2 (d6601f4ec): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.21 (92477967)
doveconf: Warning: service auth { client_limit=1000 } is lower than
required under max. load (2500)
doveconf: Warning: service anvil { client_limit=1000 } is lower than
required under max. load (1503)
# OS: Linux 4.15.0-60-generic x86_64 Ubuntu 18.04.3 LTS
auth_default_realm = **edited**
auth_master_user_separator = *
auth_mechanisms = plain login
default_process_limit = 500
first_valid_uid = 200
last_valid_uid = 200
mail_location = mbox:~/mail:INBOX=/var/mail/%u
mail_plugins = " notify replication zlib quota"
mail_privileged_group = mail
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 index ihave
duplicate mime foreverypart extracttext vnd.dovecot.debug
mdbox_rotate_size = 10 M
namespace inbox {
  inbox = yes
  location =
  mailbox "Éléments envoyés" {
    special_use = \Sent
  }
  mailbox "Éléments supprimés" {
    special_use = \Trash
  }
  mailbox Brouillon {
    special_use = \Drafts
  }
  mailbox "Courrier indésirable" {
    special_use = \Junk
  }
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix = INBOX/
  separator = /
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
passdb {
  args = /etc/dovecot/passwd.masterusers
  driver = passwd-file
  master = yes
}
plugin {
  mail_replica = tcps:**edited**:53683
  quota = count:User quota
  quota_grace = 10%%
  quota_rule = *:storage=10G
  quota_rule2 = INBOX/Trash:storage=+100M
  quota_status_nouser = DUNNO
  quota_status_overquota = 552 5.2.2 La boite courriel du destinataire est
pleine - Mailbox is full
  quota_status_success = DUNNO
  quota_vsizes = yes
  quota_warning = storage=100%% quota-warning 100 %u
  quota_warning2 = storage=95%% quota-warning 95 %u
  quota_warning3 = storage=90%% quota-warning 90 %u
  quota_warning4 = storage=85%% quota-warning 85 %u
  quota_warning5 = storage=75%% quota-warning 75 %u
  quota_warning6 = -storage=80%% quota-warning '-80' %u
  quota_warning7 = -storage=100%% quota-warning '-100' %u
  sieve = /var/vmail/domains/%Ld/%Ln/.dovecot.sieve
  sieve_before = /var/vmail/sieve/before.sieve
  sieve_default = /var/vmail/sieve/default.sieve
  sieve_extensions = +vnd.dovecot.debug
  sieve_global = /var/vmail/sieve/global
  zlib_save = lz4
}
protocols = " imap lmtp sieve"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
  unix_listener auth-userdb {
    group = vmail
    mode = 0666
    user = vmail
  }
}
service imap-login {
  inet_listener imap {
    port = 0
  }
  process_min_avail = 10
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0660
    user = postfix
  }
}
service quota-status {
  client_limit = 1
  executable = quota-status -p postfix
  inet_listener {
    address = **edited**
    port = 53684
  }
}
service quota-warning {
  executable = script /usr/local/bin/quota-warning.sh
  unix_listener quota-warning {
    user = vmail
  }
}
ssl = required
ssl_cert = </etc/ssl/**edited**/**edited**.crtbundle
ssl_client_ca_dir = /etc/ssl/certs
ssl_key =  # hidden, use -P to show it
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
protocol lmtp {
  mail_plugins = " notify replication zlib quota sieve"
  postmaster_address = postmaster@**edited**
}
protocol imap {
  mail_plugins = " notify replication zlib quota imap_zlib imap_quota"
}

-- 
Francis
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