I believe that warning message indicates that the path listed was found in the local Duplicati database twice. How that would happen I’m not sure but my GUESS would be a symlink, circular reference, or similar folder structure oddity might cause it.
As far as the implications of that, I don’t really know. Since it’s a warning and not an error I expect it’s not a major issue. I wouldn’t know what to suggest for a response to it, though if I saw that in my backups I’d probably ignore it for a few runs and see if it goes away and if not then come back here and ask if I should be worried that it hasn’t resolved itself.