Dsync deleting mailboxes due to duplicate UIDs

[Andreas Thienemann]

Hi,

I am attempting to migrate a mailspool from a cyrus server to a dovecot 
server using the dsync backup approach as described in the wiki at 
<https://wiki.dovecot.org/Migration/Dsync>.

The first attempt works great. Everything copies over and a quick glance 
over the spool looks good.

Running doveadm backup again though results in the following error:

dsync(eggs): Warning: Deleting mailbox 'INBOX.MailRestore': 
UID=39 already exists locally for a different mail: Headers hashes don't 
match (18d567fc7e258a67e47b629c8bb16500 vs 
230354b2d5cad21ebbb4a7440b977adb)

As promised, the folder MailRestore is gone after dsync finishes.

Running doveadm backup again for the third time copies the folder again.

Running it a fourth time gives the same error and the folder is gone.

Trying to figure this out I initially had a few mails where the header 
hashes were 68b329da9893e34099c7d8ad5cb9c940, the md5sum of an empty 
string.
Turns out that the source mailspool had a few broken emails and cleaning 
these out fixed most my issues.

But in this case, I am stumped. UID 39 on the server is a valid mail with 
correct headers and everything.

Is there a good way of identifying the two mails that seem to clash? I had 
a quick look at the dovecot code but did not see how the header_stream 
gets hashed into the hdr_hash used for comparing mails.

Thanks,
  Andreas