Do you have the DSON importer installed and/or are using any content from DAZ that has DSON files and Poser Companion files?

Sometime around DS3 and through DS4, DAZ decided to stop creating multiple installers.  They just tossed the Poser and DS files all in one installer (now zip) and added shortcuts to the pose files for DS to find the DS presets.

.duf format came into use with DS4.5.  I believe it stands for DAZ Unified Format. 

Newer content at DAZ if it can be used in Poser generally comes two ways: With a DS version and Poser version listed in the downloads or with the DSON zipped files and separate Poser companion files.

If you do not use the DSON or DS at all, you can delete them.  If you use the DSON to import DAZ files (some prop files do not have Poser core installers) into Poser, do not delete them.