Question regarding replication - duplicate emails

[Remko Lodder]

Dear Dovecot and community,

We run a small email service for our customers, based on two machines that are made “redundant or clustered” by using the replication feature of Dovecot.
This works well, for most emails.

Sometimes the following happends:

Email to our support database arrives at the inbox.
Every period a cronjob looks into that mailbox and parses the information and makes a support ticket from that message.
The cronjob deletes the email afterwards and sees whether there are more new mails or not.

In between the deletion and the (next) check, the email gets “resynced” again from the “other machine”, and is seen as a new
email by the cronjob and gets handled again. So basically we get two tickets for one email. After that second run the email is
no longer replicated.

This behaviour can also be seen via webmail and email clients, this suggests that the replication might not be aware (enough) that
the replication is occurring or that an item already had been synced from A to B (and does not need to get back from B to A after A
deleted it).

Is there a way to fiddle with the acknowledgement timing or give the processes some more space/time to get on par with eachother?

Below is the configuration of machine B, they are synchronised through puppet, so only the hostname and IP addresses are different.
(so for replication, A has: tcps:hostname_of_b:12346 and B has tcps:hostname_of_a:12346). Also note that HAproxy is prepared but
not in use at all.

# 2.2.33.1 (e9afa7f18): /usr/local/etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.20 (7cd71ba)
# OS: FreeBSD 11.1-RELEASE amd64
auth_mechanisms = plain login
disable_plaintext_auth = no
doveadm_password =  # hidden, use -P to show it
haproxy_trusted_networks = IPv4_Haproxy IPv6_Haproxy
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
lmtp_save_to_detail_mailbox = yes
mail_fsync = always
mail_location = sdbox:~/sdbox
mail_plugins = " quota notify replication"
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 imapsieve vnd.dovecot.imapsieve
namespace {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Spam {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
  separator = .
}
passdb {
  driver = pam
}
plugin {
  imapsieve_mailbox1_before = file:/usr/local/lib/dovecot/sieve/report-spam.sieve
  imapsieve_mailbox1_causes = COPY
  imapsieve_mailbox1_name = Spam
  imapsieve_mailbox2_before = file:/usr/local/lib/dovecot/sieve/report-ham.sieve
  imapsieve_mailbox2_causes = COPY
  imapsieve_mailbox2_from = Spam
  imapsieve_mailbox2_name = *
  mail_log_events = delete undelete expunge copy mailbox_delete mailbox_rename
  mail_log_fields = uid box msgid size
  mail_replica = tcps:hostname_of_machine_a:12346
  sieve = ~/.dovecot.sieve
  sieve_after = /usr/local/etc/dovecot/sieve/after/
  sieve_before = /usr/local/etc/dovecot/sieve/global/
  sieve_pipe_bin_dir = /usr/local/lib/dovecot/sieve
  sieve_plugins = sieve_imapsieve sieve_extprograms
  sieve_vacation_dont_check_recipient = yes
}
postmaster_address = postmaster at ourdomain.tld
protocols = imap pop3 lmtp sieve
replication_max_conns = 100
service aggregator {
  fifo_listener replication-notify-fifo {
    mode = 0666
  }
  unix_listener replication-notify {
    mode = 0666
  }
}
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service doveadm {
  inet_listener {
    port = 12346
    ssl = yes
  }
}
service imap-login {
  inet_listener imap_haproxy {
    haproxy = yes
    port = 10143
  }
  inet_listener imaps_haproxy {
    haproxy = yes
    port = 10144
    ssl = yes
  }
  service_count = 1
}
service imap {
  process_limit = 1024
}
service lmtp {
  inet_listener lmtp {
    address = IPv4, IPv6, 127.0.0.1 ::1
    port = 24
  }
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
service pop3 {
  process_limit = 1024
}
service replicator {
  process_min_avail = 1
  unix_listener replicator-doveadm {
    mode = 0666
  }
}
ssl_ca = </usr/local/etc/letsencrypt/live/host.ourdomain.tld/fullchain.pem
ssl_cert = </usr/local/etc/letsencrypt/live/host.ourdomain.tld/cert.pem
ssl_client_ca_file = /usr/local/certificates/letsencrypt-ca.pem
ssl_key =  # hidden, use -P to show it
ssl_protocols = !SSLv2 !SSLv3
userdb {
  driver = passwd
}
verbose_proctitle = yes
protocol lmtp {
  auth_username_format = %n
  mail_plugins = quota sieve replication notify
  postmaster_address = postmaster at ourdomain.tld
}
protocol lda {
  mail_plugins = " quota notify replication sieve replication notify"
}
protocol imap {
  imap_client_workarounds = delay-newmail
  mail_max_userip_connections = 50
  mail_plugins = " quota notify replication imap_quota imap_sieve"
}

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