Sources:
https://dukeworld.com/classic%20dukeworld/tc/doom/doomduke20.zip
https://dukeworld.com/classic%20dukeworld/tc/doom/doomduke21.zip

Steps:
* Unpacked doomduke20.zip's readme.txt and final20.zip,
and then unpacked final20.zip. final20.zip's doomduke.bat was deleted later.
* Unpacked doomduke21.zip and then applied the patch from doomduke21.exe.
That was patching DOOM.GRP, modifying its internal TILES000.ART file
and mistakenly also adding KEXTRACT.EXE to DOOM.GRP. The sole change to
TILES000.ART was a fix to the grenade launcher pickup sprite's background,
changed from pink to transparent (technically also pink in the palette).
* Renamed doomgame.con to GAME.CON.
* Unpacked DOOM.GRP with kextract and then deleted DOOM.GRP.
* Deleted the following DOOM.GRP files which were already
in final20.zip: DOOMDEFS.CON, DOOMGAME.CON, DOOMUSER.CON.
Also deleted KEXTRACT.EXE and KGROUP.EXE.
* Created a new archive from the available contents.