This error comes up when there are a lot of files involved. There are no missing files reported and I attempt to create a zip to get rid of the unused files, but it fails. I always take the map with all its files away to another folder in my desktop to create the res files so resguy does not involve the entire svencoop folder. So far it always worked.

ERROR:
Duplicate filename on disk:
sound\lastunic\ambience\appel.wav
sound\lastunic\ambience\appel.wav
Cleaning up... Done
Archive creation failed! Try a different option.

There are obviously no duplicates.