I copied a whole folder that did not exist in my new mail archive database from an old mail archive database into this new database. It was the whole archive for 2017, which had gone lost in my mail account during an update at the university. I set up my new mail archive database and imported all archived years in my actual mail account from 2016 to this year, but since 2017 was missing on the server, I decided to copy the whole archived year from the old mail database into the new one.
So, I did not plan to copy any duplicates.
I copied a well functioning folder.
Thanks to your hint I checked whether the 12 mails left did exist in my new database, and they did indeed exist in the 2017 folder. So, I do not know why Devonthink tried to copy these items left, and I do not know why these items were left in the old database in the first place, since they had been moved and earlier, they lived in the old database. How could they with the same UUID?
It makes me feel uncomfortable with my data.
In case you ask why I wish to set up a new database with emails, I wish to. Devonthink in earlier years screwed up many Japanese mail titles (Betreff), now this works well.
Best, problem does no longer exist but the process is a bit unclear.