@backupti, @Ferdis, @ts678, @belidzs, @przemas75:
Thanks for reporting.
The error is that Duplicati is attempting to insert the same file twice for some reason.
If one of you who did not downgrade could try to add:
--log-file-log-filter=+*.FileEntry*
--log-file=<path-to-file>
and then run the backup again to collect log data. The last line before the error should be: Skipped checking file, because no metadata was updated <path-to-error-file>
Then check in the log if that file is reported elsewhere.
Yes, that sounds like this issue: Processing files which are not in backup sources