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https://web.archive.org/web/20181208152248/https://www.introversion.co.uk/defcon/downloads/defcon-win32-v1.6.exe
Or based on my short experiment, I think you can copy/paste the game files to another location and run them locally. The authentication will fail and it'll ask if you want to play the demo version.
Then you can run the Steam version from the client and play against yourself without needing to multi-box or a second Steam account.
All the other achevements need a minimum of two human players, but you can add AI for achevements that require more than that.
You can *technically* do it yourself by downloading the demo directly from Introversion's website and running it at the same time as your steam copy. This works best with multiple computers.