Getting duplicated mails while using imapc and dsync

[Michael]

Hello list,


currently I'm migrating our old mailserver to a new mail architecture
that is based on a master-master dovecot setup (two machines).


The replication actually works but I'm also syncing our mails from the 
current mailserver via imapc on a regular schedule.


For some reason I've in most (but not all) imapc-synced mailboxes 
duplicated emails that I also can't deduplicate with doveadm.


I've also set up test-mailboxes that I don't sync with imapc and in 
those I don't have any duplicated mails.


My command to sync the mails from the old host to the master-master 
system looks as follows:

doveadm -o imapc_user=$i*dovemig -o imapc_password=<password> -o 
imapc_host=<oldhost> -o imapc_features="rfc822.size fetch-headers" -o 
mail_prefetch_count=20 -v -D sync -R -u$EMAIL imapc: &> $LOGS/$i.log


So, is this actually the wrong approach to sync mails from the old host 
to the new one or is it just not recommended to sync them over-and-over 
again in such a setup via imapc or is it better to sync them only once?

Any hint/pointing me to the right solution is greatly appreciated.


Michael