Source: https://dukeworld.com/classic%20dukeworld/tc/doom/doom31.zip

Steps:
* Unpacked doom31.zip, short of Doomduke.exe and the ICO files.
* Renamed DOOM.CON to GAME.CON.
* Unpacked DOOM.GRP with kextract and then deleted DOOM.GRP.
* Deleted the following files extracted from DOOM.GRP:
DOOM.ICO, DOOM.CON, DOOMDEFS.CON, DOOMDUKE.RTS, DOOMUSER.CON, KGROUP.EXE.
The CON files and DOOM.ICO matched in contents with the ones from doom31.zip.
DOOMDUKE.RTS was originally unusable as a part of the GRP file.
* Created a new archive from the available contents.