I don’t either, and you will probably have to isolate the problem to characterize it…
You can see information what files are seen and processed in a verbose log, e.g.
2021-05-26 09:41:05 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileEnumerationProcess-IncludingPath]: Including path as no filters matched: C:\PortableApps\Notepad++Portable\Data\Config\backup\webpages.txt@2021-05-25_092926
2021-05-26 09:41:06 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\PortableApps\Notepad++Portable\Data\Config\backup\webpages.txt@2021-05-25_092926, new: False, timestamp changed: True, size changed: True, metadatachanged: True, 5/26/2021 1:30:37 PM vs 5/26/2021 1:00:41 AM
2021-05-26 09:41:11 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FileBlockProcessor.FileEntry-ChangedFile]: File has changed C:\PortableApps\Notepad++Portable\Data\Config\backup\webpages.txt@2021-05-25_092926
You could try About → Show log → Live -->Verbose, but log-file=<path> and log-file-log-level=verbose might be an easier-to-study log. You’d be looking for files that repeat somehow, and also error details…
Even more details are available at extremely detailed log levels (e.g. profiling), but such logs get huge.