Schwertz: Double-barreled shotgun aka SSG appeared in DOOM II. Not in the original DOOM.
And these games never had bots. Very late multiplayer-oriented fan-made source-ports like Skulltag or Zandronum have more or less proper built-in bots. But they are not really bright in the department of AI, frankly speaking.
There was a hack for the vanilla engine as well, which turned some monsters into something close to the bots. They had DOOM marine look, attacked with one (and only one) of the player's weapons but an AI remained at the level of a dumb zombie.