Some weeks ago clients reported problems of mass mailings not being delivered to a part of the destinees. After doing some research it occurred that the problem lied in contacts that had been merged.
When contact A is merged in contact B, contact A gets deleted (still in the database but status 'is_deleted'). When creating a mailing both A and B seem to be added to the destinees.
When sending the mailing contact A obviously doesn't receive the mailing because 'is_deleted' but contact B also doesn't get the mailing because the mail is regarded as a duplicate mail.
After some looking around I found https://github.com/civicrm/civicrm-core/commit/48b1ff58 which solved the issue for us on multiple installs.
Now I hoped this would quickly find its way into the releases, but when I just took a look at the freshly released 5.34.0 it clearly hasn't.
The error is so obious though that I feel I must be missing something myself, so I hesitate to file a bug report...