SIS deduplication was broken in 2.2.16 and has been fixed with https://github.com/dovecot/core/commit/3b39022ea0513363241cf852b7d454c841584... but still sometimes (just several times in a month or so) dovecot creates duplicated attachments.
As you can see in directory listings below all attachments was created at (about) the same time, but one of them has different inode number.
It easy to discover using https://github.com/moisseev/doveadm-tools/blob/master/bin/dsisck
# dsisck -n
mail_uid=vmail mail_attachment_dir=/vmail/attachments
==> Checking SIS...
# ln -f ./30/b3/30b367c584a123eee59478adf3e4f4c9e1226545-c56eae04a67c3157287f01003d96bafd ./30/b3/30b367c584a123eee59478adf3e4f4c9e1226545-7ad0a411d17c31572b7a01003d96bafd # mkdir -p -m 700 ./64/8f/hashes && ln ./64/8f/648f5cfa27af6d20c8570fdcaeab997663e15105-55430d2cd4432c571cb600003d96bafd ./64/8f/hashes/648f5cfa27af6d20c8570fdcaeab997663e15105 # ln -f ./6f/3f/6f3fa3e4d374a9c80d07af54960ce0e7adb2e0fe-b1935817ed753557546700003d96bafd ./6f/3f/6f3fa3e4d374a9c80d07af54960ce0e7adb2e0fe-72da1f32e38e3557947b00003d96bafd # ln -f ./dd/d9/ddd97aa6f624d4f54968d2c4956fc3a9d796b31b-6bf7c122fefb3157ad3500003d96bafd ./dd/d9/ddd97aa6f624d4f54968d2c4956fc3a9d796b31b-90fd1415e7e53157112800003d96bafd
Unexpected objects found: 0
Attachments processed: 43500 Different attachments with similar hashes skipped: 0
Attachments deduplicated: 3 File system blocks freed up: 1800 Attachment deduplication attempts failed: 0
Hash files created: 1 Hash files creation attempts failed: 0
Hash files re-linked: 0 Hash files re-link attempts failed: 0
Orphaned hash files: 0 Invalid hash files (not a regular file): 0
# cd /vmail/attachments
# ls -li ./30/b3/ ./30/b3/hashes/ ./30/b3/: total 1996 7464083 -rw------- 1 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-7ad0a411d17c31572b7a01003d96bafd 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-bb6eae04a67c3157287f01003d96bafd 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-bd6eae04a67c3157287f01003d96bafd 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-bf6eae04a67c3157287f01003d96bafd 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-c36eae04a67c3157287f01003d96bafd 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545-c56eae04a67c3157287f01003d96bafd 7464084 drwx------ 2 vmail vmail 512 May 10 09:16 hashes
./30/b3/hashes/: total 332 7464082 -rw------- 6 vmail vmail 337719 May 10 09:16 30b367c584a123eee59478adf3e4f4c9e1226545
# ls -li ./6f/3f/ ./6f/3f/hashes/ ./6f/3f/: total 644 15088414 -rw------- 1 vmail vmail 326656 May 13 11:23 6f3fa3e4d374a9c80d07af54960ce0e7adb2e0fe-72da1f32e38e3557947b00003d96bafd 15088422 -rw------- 2 vmail vmail 326656 May 13 11:23 6f3fa3e4d374a9c80d07af54960ce0e7adb2e0fe-b1935817ed753557546700003d96bafd 15088448 drwx------ 2 vmail vmail 512 May 13 11:23 hashes
./6f/3f/hashes/: total 320 15088422 -rw------- 2 vmail vmail 326656 May 13 11:23 6f3fa3e4d374a9c80d07af54960ce0e7adb2e0fe
# ls -li ./dd/d9/ ./dd/d9/hashes/ ./dd/d9/: total 748 80548 -rw------- 3 vmail vmail 250640 May 10 18:19 ddd97aa6f624d4f54968d2c4956fc3a9d796b31b-68f7c122fefb3157ad3500003d96bafd 80548 -rw------- 3 vmail vmail 250640 May 10 18:19 ddd97aa6f624d4f54968d2c4956fc3a9d796b31b-6bf7c122fefb3157ad3500003d96bafd 80547 -rw------- 1 vmail vmail 250640 May 10 18:19 ddd97aa6f624d4f54968d2c4956fc3a9d796b31b-90fd1415e7e53157112800003d96bafd 80549 drwx------ 2 vmail vmail 512 May 10 18:19 hashes
./dd/d9/hashes/: total 248 80548 -rw------- 3 vmail vmail 250640 May 10 18:19 ddd97aa6f624d4f54968d2c4956fc3a9d796b31b
# doveconf -n # 2.2.24 (a82c823): /usr/local/etc/dovecot/dovecot.conf # Pigeonhole version 0.4.14 (099a97c) # OS: FreeBSD 10.3-RELEASE i386 auth_default_realm = example.com auth_mechanisms = digest-md5 cram-md5 plain apop doveadm_password = # hidden, use -P to show it first_valid_gid = 1000 first_valid_uid = 1000 lda_mailbox_autosubscribe = yes listen = * mail_attachment_dir = /vmail/attachments mail_gid = vmail mail_home = /vmail/%d/%n mail_location = mdbox:~/mdbox mail_plugins = quota zlib acl mail_shared_explicit_inbox = yes mail_uid = vmail 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 namespace { list = children location = mdbox:%%h/mdbox:INDEXPVT=~/mdbox/shared/%%u prefix = shared/%%u/ separator = / subscriptions = no type = shared } namespace inbox { inbox = yes location = mailbox Archives { special_use = \Archive } mailbox Drafts { special_use = \Drafts } mailbox Junk { auto = subscribe special_use = \Junk } mailbox Sent { special_use = \Sent } mailbox Trash { special_use = \Trash } prefix = separator = / } passdb { args = scheme=plain username_format=%n /usr/local/etc/dovecot/dovecot.auth/%d.passwd driver = passwd-file } plugin { acl = vfile acl_shared_dict = file:/vmail/shared-mailboxes.db antispam_backend = mailtrain antispam_mail_notspam = report_ham antispam_mail_sendmail = /usr/local/libexec/dovecot/dovecot-lda antispam_mail_sendmail_args = -d;spam@example.com;-m antispam_mail_spam = report_spam antispam_spam = Junk antispam_trash = Trash;train_ham;train_prob;train_spam quota = dict:User quota::file:%h/dovecot-quota quota_rule = *:storage=2G quota_rule2 = Trash:storage=+10%% quota_rule3 = Spam:storage=+20%% quota_status_nouser = DUNNO quota_status_success = DUNNO sieve_after = /usr/local/etc/dovecot/sieve/sieve.after sieve_before = /usr/local/etc/dovecot/sieve/sieve.before sieve_vacation_min_period = 0 zlib_save = gz zlib_save_level = 3 } postmaster_address = postmaster@example.com protocols = imap lmtp sieve pop3 quota_full_tempfail = yes service auth { unix_listener /var/spool/postfix/private/auth { group = postfix mode = 0660 user = postfix } unix_listener auth-userdb { user = vmail } } service config { unix_listener config { mode = 0600 user = vmail } } service imap-login { inet_listener imaps { port = 0 } process_limit = 200 } service lmtp { unix_listener /var/spool/postfix/private/dovecot-lmtp { group = postfix mode = 0660 user = postfix } user = vmail } service pop3-login { inet_listener pop3s { port = 0 } } service quota-status { client_limit = 1 executable = quota-status -p postfix unix_listener /var/spool/postfix/private/quota-status { user = postfix } } ssl_cert =