A duk file extension is related to the Duck FMV video file format. Probably obsolete.
DUK file extension - Duck FMV video
What is duk file? How to open duk files?
File type specification:
The duk file extension was associated with the multimedia codec developed by Duck Corporation, later owned by On2 Technologies, which was acquired by Google in 2010.
The duk file contains FMV video data encoded by Duck codec.
Duck codec was probably popular on video gaming consoles, such as 3DO.
This duk file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.