Restoring mailboxes from backup duplicates messages in POP clients

[Tanstaafl]

On Wed Apr 10 2019 09:13:41 GMT-0400 (Eastern Standard Time), Luis F. V.
Gomes via dovecot <dovecot at dovecot.org> wrote:
> I had a disk problem and had to reformat it. All mailboxes were backed 
> up using rsync.
> After I restored the mailboxes, the POP clients (Thunderbird) that 
> were configured to leave the messages on the mailserver for, let's 
> say, 30 days, didn't understand that some messages were already 
> transfered and the users got duplicated messages in their Inbox.
> How can we avoid this?

Don't use rsync, use the built in dovecot backup capability?